SDK / exoplayer

SingleFrameGlTextureProcessor is now an abstract class containing a
default implementation of the more flexible GlTextureProcessor interface
while still exposing the same simple abstract methods for single frame
processing it previously did.

FrameProcessorChain and GlEffect will be changed to use
GlTextureProcessor in follow-ups.

PiperOrigin-RevId: 453633000
(cherry picked from commit 457f4461)