[paint] Use tags for image formats

This fits better with the rest of the
HarfBuzz API.

src/hb-ot-color-cbdt-table.hh

@@ -947,7 +947,7 @@ struct CBDT
       if (unlikely (!get_extents (font, glyph, &extents)))
         return false;
 
-      funcs->image (data, blob, "image/png", &extents);
+      funcs->image (data, blob, HB_PAINT_IMAGE_FORMAT_PNG, &extents);
 
       hb_blob_destroy (blob);
       return true;

src/hb-ot-color-sbix-table.hh

@@ -248,7 +248,7 @@ struct sbix
       if (!get_extents (font, glyph, &extents))
         return false;
 
-      funcs->image (data, blob, "image/png", &extents);
+      funcs->image (data, blob, HB_PAINT_IMAGE_FORMAT_PNG, &extents);
 
       hb_blob_destroy (blob);
       return true;

src/hb-ot-color-svg-table.hh

@@ -103,7 +103,7 @@ struct SVG
       if (blob == hb_blob_get_empty ())
         return false;
 
-      funcs->image (data, blob, "image/svg+xml", nullptr);
+      funcs->image (data, blob, HB_PAINT_IMAGE_FORMAT_SVG, nullptr);
 
       hb_blob_destroy (blob);
       return true;

src/hb-paint.cc

@@ -71,7 +71,7 @@ hb_paint_color_nil (hb_paint_funcs_t *funcs, void *paint_data,
 static void
 hb_paint_image_nil (hb_paint_funcs_t *funcs, void *paint_data,
                     hb_blob_t *image,
-                    const char *mimetype,
+                    hb_tag_t format,
                     hb_glyph_extents_t *extents,
                     void *user_data) {}
 
@@ -347,10 +347,10 @@ hb_paint_color (hb_paint_funcs_t *funcs, void *paint_data,
 void
 hb_paint_image (hb_paint_funcs_t *funcs, void *paint_data,
                 hb_blob_t *image,
-                const char *mimetype,
+                hb_tag_t format,
                 hb_glyph_extents_t *extents)
 {
-  funcs->image (paint_data, image, mimetype, extents);
+  funcs->image (paint_data, image, format, extents);
 }
 
 void

src/hb-paint.h

@@ -222,12 +222,26 @@ typedef void (*hb_paint_color_func_t) (hb_paint_funcs_t *funcs,
                                        float alpha,
                                        void *user_data);
 
+/**
+ * HB_PAINT_IMAGE_FORMAT_PNG:
+ *
+ * Tag identifying png images in #hb_paint_image_func_t callbacks.
+ */
+#define HB_PAINT_IMAGE_FORMAT_PNG HB_TAG('p','n','g',' ')
+
+/**
+ * HB_PAINT_IMAGE_FORMAT_SVG:
+ *
+ * Tag identifying svg images in #hb_paint_image_func_t callbacks.
+ */
+#define HB_PAINT_IMAGE_FORMAT_SVG HB_TAG('s','v','g',' ')
+
 /**
  * hb_paint_image_func_t:
  * @funcs: paint functions object
  * @paint_data: The data accompanying the paint functions
  * @image: the image data
- * @mimetype: the mime type for the image data
+ * @format: the image format as a tag
  * @extents: (nullable): glyph extents
  * @user_data: user data passed to the hb_font_paint_glyph() call
  *
@@ -235,17 +249,19 @@ typedef void (*hb_paint_color_func_t) (hb_paint_funcs_t *funcs,
  * glyph image.
  *
  * This method is intended for glyphs with image blobs in the CBDT,
- * sbix or SVG tables. The @mimetype identifies the kind of data
+ * sbix or SVG tables. The @format identifies the kind of data that
- * that is contained in @image. Possible values include "image/png"
+ * is contained in @image. Possible values include #HB_PAINT_IMAGE_FORMAT_PNG
- * and "image/svg+xml". The glyph extents are provided if available,
+ * and HB_PAINT_IMAGE_FORMAT_SVG.
- * and should be used to position the image.
+ *
+ * The glyph extents are provided if available, and should be used
+ * to position the image.
  *
  * Since: REPLACEME
  */
 typedef void (*hb_paint_image_func_t) (hb_paint_funcs_t *funcs,
                                        void *paint_data,
                                        hb_blob_t *image,
-                                       const char *mimetype,
+                                       hb_tag_t format,
                                        hb_glyph_extents_t *extents,
                                        void *user_data);
 
@@ -693,7 +709,7 @@ hb_paint_color (hb_paint_funcs_t *funcs, void *paint_data,
 HB_EXTERN void
 hb_paint_image (hb_paint_funcs_t *funcs, void *paint_data,
                 hb_blob_t *image,
-                const char *mimetype,
+                hb_tag_t format,
                 hb_glyph_extents_t *extents);
 
 HB_EXTERN void

src/hb-paint.hh

@@ -97,10 +97,10 @@ struct hb_paint_funcs_t
                 !user_data ? nullptr : user_data->color); }
   void image (void *paint_data,
               hb_blob_t *image,
-              const char *mimetype,
+              hb_tag_t format,
               hb_glyph_extents_t *extents)
   { func.image (this, paint_data,
-                image, mimetype, extents,
+                image, format, extents,
                 !user_data ? nullptr : user_data->image); }
   void linear_gradient (void *paint_data,
                         hb_color_line_t *color_line,

util/hb-test.c

@@ -325,7 +325,7 @@ static void
 paint_image (hb_paint_funcs_t *funcs,
              void *paint_data,
              hb_blob_t *blob,
-             const char *mimetype,
+             hb_tag_t format,
              hb_glyph_extents_t *extents,
              void *user_data)
 {
@@ -335,7 +335,7 @@ paint_image (hb_paint_funcs_t *funcs,
   cairo_pattern_t *pattern;
   cairo_matrix_t m;
 
-  if (strcmp (mimetype, "image/png") != 0)
+  if (format != HB_PAINT_IMAGE_FORMAT_PNG)
     return;
 
   r.blob = blob;