Sprite: Perform CPU visibility culling
MRI-Binding: Add 'ViewportElement.viewport=' binding
Graphics: Fix viewport effect rendering bug

We didn't account for the spec dictating that
scissor test does affect FBO blit operations,
which resulted in corrupted output if more than
one viewport with an active effect (tone, color)
was created.

The reason I never caught this before must be
that the fglrx-legacy driver is actually bugged
in this aspect and ignores the scissor on blit.
Graphics: Remove useless and confusing function
Audio: Fix BGM not starting after ME has ended

A previous commit prevented the MeWatch from
starting a BGM stream that was in stopped state.
However, when a new BGM is loaded while the ME
is still playing, the BGM stream will be stopped
even though we want it to start after the ME
finishes.

Also add some comments trying to explain members
of 'AudioStream' a bit better.
Remove completely outdated note