Don't decimate FBOs if none were used.

Not sure why this is happening, but fixes #1260.
Don't go decimate happy on FBOs if a game reuses.

Some games, e.g. games that render at < 60fps, reuse FBOs.
This was making all three pointers the same, making us more likely
to decimate things when we ought not to.
Merge pull request #1261 from unknownbrackets/fbo-flicker

Fix FBO decimation in a cleaner way