Fix average scene luminance

Average scene luminance is calculated using hardware mipmaps. This gives incredibly inaccurate results since we're operating on a (usually) non-Po2 texture. The easiest solution is to resize the tender target to be Po2.

Additionally, wrong UV coordinates are being used in the luminance shader when sampling from original scene. This results in only a portion of the scene making into the FBO which is used to generate the mipmaps.