Sprite: Add special case shader for translucent effect

Translucent sprites (opacity < 255) are very common,
so using a custom shader instead of the full blown effect one
helps a lot, especially on mobile.

mkxp.pro

@@ -193,6 +193,7 @@ EMBED = \
 	shader/simple.frag \
 	shader/simpleColor.frag \
 	shader/simpleAlpha.frag \
+	shader/simpleAlphaUni.frag \
 	shader/flashMap.frag \
 	shader/simple.vert \
 	shader/simpleColor.vert \