renderer: support rotated playback

This was a nightmare to implement, but the current solution seems
relatively robust. 180° rotations are trivial, only 90° rotations
require special handling. The implementation is based on the new ability
to *transpose* pl_shader outputs during dispatching.

To make all of the aspect ratio and scaling code do the right thing, the
basic premise is that, very early during rendering, we "fix" the
dst_rect by un-rotating it. This way, all of the scalers (and user
shaders etc.) see the "correct" dst rect, as though the target texture
were itself somehow magically rotated.

This normalization is only undone in the very last step during output to
the target planes. This implementation is also transparent to
interpolation, which always sees "un-rotated" planes.

Note that the current implementation just completely ignores overlays,
which probably break horribly under rotation. I'll fix that in a future
commit, when I can reason better about what the correct behavior is
supposed to be.

Closes #144