[core][ios][android][macos] Use premultiplied image directly for RasterTile and ImageSource, un-premultiply in the shader for blending

src/mbgl/shaders/raster.cpp

@@ -45,6 +45,12 @@ void main() {
     // read and cross-fade colors from the main and parent tiles
     vec4 color0 = texture2D(u_image0, v_pos0);
     vec4 color1 = texture2D(u_image1, v_pos1);
+    if (color0.a > 0.0) {
+        color0.rgb = color0.rgb / color0.a;
+    }
+    if (color1.a > 0.0) {
+        color1.rgb = color1.rgb / color1.a;
+    }
     vec4 color = mix(color0, color1, u_fade_t);
     color.a *= u_opacity;
     vec3 rgb = color.rgb;