Fix issues with float inaccuracy on 'clipping' overlay

* There was a problem where a value of 1.0 would show up as 'clipping' if
  the range meant that it was transformed to something like 1.0000000432f
  so we use FLT_EPSILON as a slight fudge factor.
Implement NaN/inf/-ve and clipping overlays on OpenGL
Don't use FetchTexture in GLReplay, as overlay texture doesn't have it

* Instead of using FetchTexture we store the internal format in the texture
  data in the driver, so it's available for use.
* This fixes some issues with the overlay textures