Don't skip alpha in 565/clut framebuffers textures.
Change CheckAlpha() to apply to any pixel data.

For example, cluts.  But not yet, just changing it here.
Detect if framebuffer clut uses alpha.

It's not a perfect check but it may allow skipping alphatests sometimes.
Merge pull request #6221 from unknownbrackets/framebuffer-clut

When rendering a framebuffer using clut, check alpha