Textures and renderbuffers were storing both the OpenGL internalformat
and the SwiftShader implementation format (i.e. sw::Format). This
change removes the implementation format, only keeping it in the Image
class.

Change-Id: Ie6ac96f6450b9a55ea9b49c6cf4b2c0681e95522
Reviewed-on: https://swiftshader-review.googlesource.com/14528Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>