reduce SDL2_Image's dependency chain

For the same reasons as outlined in fabiangreffrath@53ce004, this commit reduces SDL2_Image's dependency chain to only one hard dependency on libpng.

While PNG support is absolutely expected from an image processing library, support for TIFF, JPEG and WEBP is demoted to soft dependencies which rely on the corresponding libraries being installed and available to get opened at run-time.

Author: fabiangreffrath

mingw-w64-SDL2_image/PKGBUILD

@@ -28,10 +28,7 @@ build() {
     --host=${MINGW_CHOST} \
     --enable-static \
     --enable-shared \
-    --disable-jpg-shared \
-    --disable-png-shared \
-    --disable-tif-shared \
-    --disable-webp-shared
+    --disable-png-shared
 
   make
 }