Added passes for premultiplied alpha, fixed alpha blending for native transparency

shaders/util/premultiply.frag

@@ -0,0 +1,15 @@
+
+#request nativeonly true
+
+#request uniform "prev" tex
+uniform sampler2D tex;
+
+out vec4 fragment;
+in vec4 gl_FragCoord;
+
+void main() {
+    fragment = texelFetch(tex, ivec2(gl_FragCoord.x, gl_FragCoord.y), 0);
+    #if PREMULTIPLY_ALPHA > 0
+    fragment.rgb *= fragment.a;
+    #endif
+}