Please export a uniform with core aspect ratio to shaders

[kokoko3k]

When developing a shader that wants to use lateral borders to draw bezels, ambient light and so on, retroarch aspect must be set to something not "core provided".
Since the shader has no clue what is the intended aspect ratio (DAR), it has to do some, maybe wrong, guess about it by reading Originalsize, but this is far from ideal.
I propose that the core provided aspect ratio is exported as a Uniform to the shader as a float.