Detect game resolution and aspect ratio, then initialize post-processing surface to those values

[BinToss]

I noticed a snippet of the post-processing backend initializing first to 4:3 ratio and then to 16:9.

1. This may be the cause of the resolution-switching crashes
2. dgVoodoo and Chimera HATE this. It breaks their resolution overrides and crashes the Halo CE.
3. 21:9 monitors are more common than they used to be. Initializing to a 16:9 ratio could cause issues.
4. I wonder if this is what screws up the mouse cursor bound in the main menu...