gpu: Introduce gsk_texture() shader function/macro

Due to GLES and old GL not allowing non-constant texture array
lookups,we need to turn the array lookup into a big switch statementin
those versions, and that requires putting the texture() call into that
switch.

But with that trick, we can use texture IDs in GLSL.

gsk/gpu/shaders/common-gl.glsl

@@ -52,23 +52,64 @@ gsk_get_float (uint id)
   return gsk_get_float (int (id));
 }
 
-#define gsk_get_texture(id) textures[id]
 #define gsk_get_int(id) (floatBitsToInt(gsk_get_float(id)))
 #define gsk_get_uint(id) (floatBitsToUint(gsk_get_float(id)))
 
 #ifdef GSK_GLES
-void
-gsk_set_output_color (vec4 color)
+vec4
+gsk_texture (uint id,
+             vec2 pos)
 {
-  gl_FragColor = color;
+  switch(id)
+  {
+    case 0u:
+      return texture (textures[0], pos);
+    case 1u:
+      return texture (textures[1], pos);
+    case 2u:
+      return texture (textures[2], pos);
+    case 3u:
+      return texture (textures[3], pos);
+    case 4u:
+      return texture (textures[4], pos);
+    case 5u:
+      return texture (textures[5], pos);
+    case 6u:
+      return texture (textures[6], pos);
+    case 7u:
+      return texture (textures[7], pos);
+    case 8u:
+      return texture (textures[8], pos);
+    case 9u:
+      return texture (textures[9], pos);
+    case 10u:
+      return texture (textures[10], pos);
+    case 11u:
+      return texture (textures[11], pos);
+    case 12u:
+      return texture (textures[12], pos);
+    case 13u:
+      return texture (textures[13], pos);
+    case 14u:
+      return texture (textures[14], pos);
+    case 15u:
+      return texture (textures[15], pos);
+    default:
+      return vec4 (1.0, 0.0, 0.8, 1.0);
+  }
 }
-#else
+
+#else /* !GSK_GLES */
+
+#define gsk_texture(id, pos) texture (textures[id], pos)
+
+#endif
+
 layout(location = 0) out vec4 out_color;
 void
 gsk_set_output_color (vec4 color)
 {
   out_color = color;
 }
-#endif
 
 #endif

gsk/gpu/shaders/common-vulkan.glsl

@@ -32,7 +32,7 @@ layout(set = 1, binding = 0) readonly buffer FloatBuffers {
 
 layout(location = 0) out vec4 out_color;
 
-#define gsk_get_texture(id) textures[nonuniformEXT (id)]
+#define gsk_texture(id, pos) texture (textures[nonuniformEXT (id)], pos)
 #define gsk_get_buffer(id) buffers[nonuniformEXT (id)]
 #define gsk_get_float(id) gsk_get_buffer(0).floats[id]
 #define gsk_get_int(id) (floatBitsToInt(gsk_get_float(id)))

gsk/gpu/shaders/gskgpucolorize.glsl

@@ -39,7 +39,7 @@ void
 run (out vec4 color,
      out vec2 position)
 {
-  float alpha = texture (gsk_get_texture (_tex_id), _tex_coord).a * rect_coverage (_rect, _pos);
+  float alpha = gsk_texture (_tex_id, _tex_coord).a * rect_coverage (_rect, _pos);
   color = _color * alpha;
   position = _pos;
 }

gsk/gpu/shaders/gskgputexture.glsl

@@ -36,7 +36,7 @@ void
 run (out vec4 color,
      out vec2 position)
 {
-  color = texture (gsk_get_texture (_tex_id), _tex_coord) *
+  color = gsk_texture (_tex_id, _tex_coord) *
           rect_coverage (_rect, _pos);
   position = _pos;
 }

gsk/gpu/shaders/pattern.glsl

@@ -119,7 +119,7 @@ glyphs_pattern (inout uint reader,
 
       float coverage = rect_coverage (glyph_bounds, pos);
       if (coverage > 0.0)
-        opacity += coverage * texture (gsk_get_texture (tex_id), (pos - push.scale * tex_rect.xy) / (push.scale * tex_rect.zw)).a;
+        opacity += coverage * gsk_texture (tex_id, (pos - push.scale * tex_rect.xy) / (push.scale * tex_rect.zw)).a;
     }
 
   return color * opacity;
@@ -132,7 +132,7 @@ texture_pattern (inout uint reader,
   uint tex_id = read_uint (reader);
   vec4 tex_rect = read_vec4 (reader);
 
-  return texture (gsk_get_texture (tex_id), (pos - push.scale * tex_rect.xy) / (push.scale * tex_rect.zw));
+  return gsk_texture (tex_id, (pos - push.scale * tex_rect.xy) / (push.scale * tex_rect.zw));
 }
 
 vec4