Fix issue #435: PixelEffects have flipped screen_coords.y.

src/scripts/graphics.lua

@@ -1290,6 +1290,7 @@ do
 		#define Image sampler2D
 		#define extern uniform
 		#define Texel texture2D
+		layout(origin_upper_left) in vec4 gl_FragCoord;
 		uniform sampler2D _tex0_;]]
 		local footer = [[void main() {
 			// fix weird crashing issue in OSX when _tex0_ is unused within effect()

src/scripts/graphics.lua.h

@@ -6273,6 +6273,9 @@ const unsigned char graphics_lua[] =
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