[core][ios][osx][android] make SpriteImage accept PremultipliedImage

the SpriteImage constructor signature changes from

SpriteImage(
        uint16_t width, uint16_t height, float pixelRatio,
        std::string&& data, bool sdf = false);

to

SpriteImage(PremultipliedImage&&, float pixelRatio, bool sdf = false)

include/mbgl/sprite/sprite_image.hpp

@@ -3,6 +3,7 @@
 
 #include <mbgl/util/noncopyable.hpp>
 #include <mbgl/util/geo.hpp>
+#include <mbgl/util/image.hpp>
 
 #include <string>
 #include <memory>
@@ -12,27 +13,18 @@ namespace mbgl {
 
 class SpriteImage : private util::noncopyable {
 public:
-    SpriteImage(
-        uint16_t width, uint16_t height, float pixelRatio, std::string&& data, bool sdf = false);
+    SpriteImage(PremultipliedImage&&, float pixelRatio, bool sdf = false);
 
-    // Logical dimensions of the sprite image.
-    const uint16_t width;
-    const uint16_t height;
+    PremultipliedImage image;
 
     // Pixel ratio of the sprite image.
     const float pixelRatio;
 
-    // Physical dimensions of the sprite image.
-    const uint16_t pixelWidth;
-    const uint16_t pixelHeight;
-
-    // A string of an RGBA8 representation of the sprite. It must have exactly
-    // (width * ratio) * (height * ratio) * 4 (RGBA) bytes. The scan lines may
-    // not have gaps between them (i.e. stride == 0).
-    const std::string data;
-
     // Whether this image should be interpreted as a signed distance field icon.
     const bool sdf;
+
+    float getWidth() const { return image.width / pixelRatio; }
+    float getHeight() const { return image.height / pixelRatio; }
 };
 
 } // namespace mbgl