ax::Image converts gray + alpha PNGs to RGBA.

Prior to this commit these textures where mapped to PixelFormat::RG8
textures. This did not work well as some textures, especially
monochrome black + transparency, were displayed as green +
transparency. This commit instead converts the image to RGBA using
libpng's API.

core/platform/Image.cpp

@@ -1221,6 +1221,11 @@ bool Image::initWithPngData(uint8_t* data, ssize_t dataLen)
             bit_depth = 8;
             png_set_expand_gray_1_2_4_to_8(png_ptr);
         }
+        // Expand gray + alpha to RGBA.
+        if (color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
+        {
+          png_set_gray_to_rgb(png_ptr);
+        }
         // expand any tRNS chunk data into a full alpha channel
         if (png_get_valid(png_ptr, info_ptr, PNG_INFO_tRNS))
         {
@@ -1246,9 +1251,6 @@ bool Image::initWithPngData(uint8_t* data, ssize_t dataLen)
         case PNG_COLOR_TYPE_GRAY:
             _pixelFormat = backend::PixelFormat::R8;
             break;
-        case PNG_COLOR_TYPE_GRAY_ALPHA:
-            _pixelFormat = backend::PixelFormat::RG8;
-            break;
         case PNG_COLOR_TYPE_RGB:
             _pixelFormat = backend::PixelFormat::RGB8;
             break;
@@ -1285,7 +1287,7 @@ bool Image::initWithPngData(uint8_t* data, ssize_t dataLen)
         png_read_end(png_ptr, nullptr);
 
         // premultiplied alpha for RGBA8888
-        if ((color_type == PNG_COLOR_TYPE_RGB_ALPHA) || (color_type == PNG_COLOR_TYPE_GRAY_ALPHA))
+        if (color_type == PNG_COLOR_TYPE_RGB_ALPHA)
         {
             if (PNG_PREMULTIPLIED_ALPHA_ENABLED)
             {
@@ -2588,26 +2590,14 @@ bool Image::saveImageToJPG(std::string_view filePath)
 void Image::premultiplyAlpha()
 {
 #if AX_ENABLE_PREMULTIPLIED_ALPHA
-    AXASSERT((_pixelFormat == backend::PixelFormat::RGBA8)
-             || (_pixelFormat == backend::PixelFormat::RG8),
-              "The pixel format should be RGBA8888 or RG88.");
+    AXASSERT(_pixelFormat == backend::PixelFormat::RGBA8,
+              "The pixel format should be RGBA8888.");
 
-    if (_pixelFormat ==  backend::PixelFormat::RGBA8) {
-        unsigned int* fourBytes = (unsigned int*)_data;
-        for (int i = 0; i < _width * _height; i++)
-        {
-            uint8_t* p   = _data + i * 4;
-            fourBytes[i] = AX_RGB_PREMULTIPLY_ALPHA(p[0], p[1], p[2], p[3]);
-        }
-    }
-    else
+    unsigned int* fourBytes = (unsigned int*)_data;
+    for (int i = 0; i < _width * _height; i++)
     {
-        uint16_t* twoBytes = (uint16_t*)_data;
-        for (int i = 0; i < _width * _height; i++)
-        {
-            uint8_t* p   = _data + i * 2;
-            twoBytes[i] = ((p[0] * p[1] + 1) >> 8) | (p[1] << 8);
-        }
+        uint8_t* p   = _data + i * 4;
+        fourBytes[i] = AX_RGB_PREMULTIPLY_ALPHA(p[0], p[1], p[2], p[3]);
     }
 
     _hasPremultipliedAlpha = true;