If we support EXT_color_buffer_float, we should also allow blits to
and from float and fixed point data, according to the extension
spec.

BUG=angleproject:1243
TEST=dEQP-GLES3.functional.fbo.blit.*

Change-Id: I0957ac0647c94e38ebd67480c608eeb16c82dbf4
Reviewed-on: https://chromium-review.googlesource.com/316030
Tryjob-Request: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>