Add support for BGRA glyphs and scaling

Scaling is done by averaging all relevant pixel, which gives much better result than nearest pixel sampling while staying simple enough and not too computationally expensive.

This enables color emoji rendering support.

I tested this on st and dmenu, both work fine once fontconfig is setup to prepend color emoji fonts. rxvt-unicode requires a small patch.

dmenu:

urxvt:

st:

src/xftfreetype.c

@@ -523,7 +523,7 @@ XftFontInfoFill (Display *dpy, _Xconst FcPattern *pattern, XftFontInfo *fi)
     /*
      * Compute glyph load flags
      */
-    fi->load_flags = FT_LOAD_DEFAULT;
+    fi->load_flags = FT_LOAD_DEFAULT | FT_LOAD_COLOR;
 
 #ifndef XFT_EMBEDDED_BITMAP
 #define XFT_EMBEDDED_BITMAP "embeddedbitmap"
@@ -775,6 +775,7 @@ XftFontOpenInfo (Display	*dpy,
     FcChar32		hash_value;
     FcChar32		rehash_value;
     FcBool		antialias;
+    FcBool		color;
     int			max_glyph_memory;
     int			alloc_size;
     int			ascent, descent, height;
@@ -831,12 +832,18 @@ XftFontOpenInfo (Display	*dpy,
     if (!(face->face_flags & FT_FACE_FLAG_SCALABLE))
 	antialias = FcFalse;
 
+    color = FT_HAS_COLOR(face) ? FcTrue : FcFalse;
+
     /*
      * Find the appropriate picture format
      */
     if (fi->render)
     {
-	if (antialias)
+	if (color)
+	{
+	    format = XRenderFindStandardFormat (dpy, PictStandardARGB32);
+	}
+	else if (antialias)
 	{
 	    switch (fi->rgba) {
 	    case FC_RGBA_RGB:
@@ -854,7+861,7 @@
 	{
 	    format = XRenderFindStandardFormat (dpy, PictStandardA1);
 	}
 
 	if (!format)
 	    goto bail2;
     }
@@ -968,7+975,7 @@
      * which doesn't happen in XftFontInfoFill
      */
     font->info.antialias = antialias;
+
+    /*
+     * Set color value, which is only known once the
+     * font was loaded
+     */
+    font->info.color = color;
+
     /*
      * bump XftFile reference count
      */