Effects render upside down when source texture is flipped

[stuartcarnie]

When setting sourceTextureIsFlipped, such as OpenGL backed cores in OpenEmu:

https://github.com/OpenEmu/OpenEmuKit/blob/750dfb1b5b679ef3099a6eea1269167c2287ddb5/Source/OpenEmuHelperApp.m#L219

the final output is correct, however, the effects are flipped: