VideoCommon: Add option to disable drawing.

Intended for performance benchmarking: skips drawing anything to the
screen. With this enabled, we still run the FIFO code/vertex loaders/etc.,
but make basically no calls to the driver/hardware.

Texture decoding is skipped in "disable drawing" mode; it's slightly
unfortunate, but I figured it's more important to skip uploading the
texture to the GPU, and the two can't be separated easily.

If you're using this option, you probably want to disable EFB and XFB
copies, and disable EFB access from CPU.

I put this patch together at delroth's request, we discussed it on IRC a few days ago.