Neither GL_EXT_texture_format_BGRA8888 nor
GL_APPLE_texture_format_BGRA8888 make mention of GL_BGRA_EXT
or GL_BGRA8_EXT being accepted as the internalformat parameter of
glCopyTexImage2D, but there's a reasonable assumption that textures
with BGRA format can be used as the read color buffer.
GL_EXT_texture_format_BGRA8888 does mention that the format is
color-renderable, and GL_APPLE_texture_format_BGRA8888 adds it to table
3.2 (but fails to mention changes to table 3.16).

Bug swiftshader:96

Change-Id: I6d18faeaded73164a2c6af87ae6109fc77825964
Reviewed-on: https://swiftshader-review.googlesource.com/17448Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>