Re-implement GL_APPLE_texture_format_BGRA8888.

Re-implement GL_APPLE_texture_format_BGRA8888.

GL_APPLE_texture_format_BGRA8888 specifies that when using the BGRA_EXT
format, it "must have an RGBA internal format". Note that
GL_EXT_texture_format_BGRA8888 also allows BGRA_EXT as internalformat.

Change-Id: I27e032054240efe96c32e27ebbdfb3c28213b1d8
Reviewed-on: https://swiftshader-review.googlesource.com/16248
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>