Move rendering buffers pointers computations inside FontRendererBlendGammaSubpixel

The logic is to disentangle the code between renderer.c and FontRenderer
so that this latter has a simple, logical interface.

lib/font_renderer/font_renderer.h

@@ -7,12 +7,19 @@
 extern "C" {
 #endif
 
-// Mirrors stbtt_bakedchar.
 typedef struct {
   unsigned short x0, y0, x1, y1;
   float xoff, yoff, xadvance;
 } GlyphBitmapInfo;
 
+// Important: when a subpixel scale is used the width below will be the width in logical pixel.
+// As each logical pixel contains 3 subpixels it means that the 'pixels' pointer
+// will hold enough space for '3 * width' uint8_t values.
+typedef struct {
+  uint8_t *pixels;
+  int width, height;
+} FontRendererBitmap;
+
 struct FontRendererImpl;
 typedef struct FontRendererImpl FontRenderer;
 
@@ -26,6 +33,10 @@ typedef struct {
     uint8_t r, g, b;
 } FontRendererColor;
 
+typedef struct {
+  int left, top, right, bottom;
+} FontRendererClipArea;
+
 FontRenderer *  FontRendererNew(unsigned int flags, float gamma);
 
 void            FontRendererFree(FontRenderer *);
@@ -39,17 +50,20 @@ int             FontRendererBakeFontBitmap(FontRenderer *, int font_height,
                     int first_char, int num_chars, GlyphBitmapInfo *glyph_info,
                     int subpixel_scale);
 
+#if 0
+// FIXME: this function needs to be updated to match BlendGammaSubpixel.
 void            FontRendererBlendGamma(FontRenderer *,
                     uint8_t *dst, int dst_stride,
                     uint8_t *src, int src_stride,
                     int region_width, int region_height,
                     FontRendererColor color);
+#endif
 
-void            FontRendererBlendGammaSubpixel(FontRenderer *,
-                    uint8_t *dst, int dst_stride,
-                    uint8_t *src, int src_stride,
-                    int region_width, int region_height,
-                    FontRendererColor color);
+void            FontRendererBlendGammaSubpixel(FontRenderer *font_renderer,
+                    FontRendererClipArea *clip, int x, int y,
+                    uint8_t *dst, int dst_width,
+                    const FontRendererBitmap *glyphs_bitmap,
+                    const GlyphBitmapInfo *glyph, FontRendererColor color);
 
 #ifdef __cplusplus
 }