VideoCommon: Add fallback handling for bounding box when disabled or unsupported

[Techjar]

The SDK seems to write "default" bounding box values before every draw (1023 0 1023 0 are the only values encountered so far, which happen to be the extents allowed by the BP registers) to reset the registers for comparison in the pixel engine, and presumably to detect whether GX has updated the registers with real values. Handling these writes and returning them on read when bounding box emulation is disabled or unsupported, even without computing real values from rendering, seems to prevent games from corrupting memory or crashing.

This obviously does not fix any effects that rely on bounding box emulation, but having the game not clobber its own code/data or just outright crash is a definite improvement.

[JMC47]

This makes it so we can run Bounding Box titles without them crashing if Bounding Box is disabled, instead just breaking Visual Effects. It's actually really useful for examining what they do.

[phire]

If these "default values" have been written by the sdk before this draw call, then we would have received them in SetBoundingBox.

Rather than hardcoded these "default values" if the desired result is to preserve what was written, then we should modify how BBdisabled works so it accepts writes and just never fetches new results from the host gpu.