It is now possible to use floating point renderbuffers and read
the data back from them. The changes include:
- Modified glReadPixels so that it always uses the blitter to
  copy the data to the external buffer.
- Added new types to both Framebuffer and some utility functions.
- Added the new ValidReadPixelsFormatType function to validate the
  glReadPixels format/type combo, which had a bit more
  possibilities than the RGBA/UNSIGNED BYTE combo previously used.

Change-Id: I1726ea57c4f7aa85bf0ffa7f323dc6a16abc34ff
Reviewed-on: https://swiftshader-review.googlesource.com/4260Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>