Keep track of whether a texture has alpha.

This way we know what sorts of blending we even have to do.  Commonly,
textures don't have any real alpha.

Not yet used for anything.
Ignore texfunc RGBA when texture is full alpha.

This allows us to skip some work in the fragment shader.
Detect clears, replace with glClear(). Needs more testing, disabled for now.
Clean up some comments.
Merge pull request #1727 from unknownbrackets/perf

Keep track of whether textures have alpha values or not
Make some logging VERBOSE
Show an error message instead of crashing on demos we don't have keys for
Only add stream data when it need more atrac data.

Tested with Djmax Portable.
Merge pull request #1731 from oioitff/atrac-fix

Only add stream data when it need more atrac data.
Merge remote-tracking branch 'upstream/master' into fps2