- Added a blendable check to InternalFormat
- Set the blendable check to the same as the Renderbuffer check for
  most formats, except true integer formats and 32F formats
- True integer formats never support blending
- 32F formats support blending based on the GL_EXT_float_blend

Bug: angleproject:4291
Change-Id: Icffdd3e6d8791303bdfdad9ec01cfd9a1d511af0
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1999495Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Alexis Hétu <sugoi@chromium.org>