This is in preparation of using it to clear single texture.
Also includes minor cleanups about not using tex target in
assert and adding enum operators.
This way it is way clearer what each viewport state is. There is
no more save and reset. The scissor test is also saved per
framebuffer.
The only rule to remember is that the viewport state (size and
origin) is reset for both the viewport and scissor when a texture
is attached or detached from an attachment slot.
This is related to the Vulkan port T68990.
This is a full cleanup of the Framebuffer module and a separation
of OpenGL related functions.
There is some changes with how the default framebuffers are handled.
Now the default framebuffers are individually wrapped inside special
GLFrameBuffers. This make it easier to keep track of the currently bound
framebuffer state and have some specificity for operations on these
framebuffers.
Another change is dropping the optimisation of only configuring the
changed attachements during framebuffers update. This does not give
any benefits and add some complexity to the code. This might be brought
back if it has a performance impact on some systems.
This also adds support for naming framebuffers but it is currently not
used.
