GLES3 changes for readpix/teximage/FBO

v1:
 * various 2101010 texture tweaks
 * integer/signed-int/unsigned-int texture tweaks
 * readpix/teximage/copyteximage ES3 error checks
 * additional FBO completeness checks for GLES
 * tested with i965+gen7
 * crash=>pass: gles3-gtf/packed_pixels/packed_pixels
 * crash=>pass: gles3-gtf/packed_pixels/packed_pixels_pbo
 * crash=>pass: gles3-gtf/packed_pixels/packed_pixels_pixelstore
 * pass=>fail: gles3-gtf/half_float/half_float_textures
   - suspect test mismatch with spec
 * no change: piglit/quick-driver