internal format is then automatically deduced. Only a few 8-bit RGB
or RGBA formats are supported for now.

hardcoded and only the fractal demo uses it, but everything will eventually switch.

Shader::SetUniform override, and implement most of the FBO code for
the Direct3D backend.

needs to know about Direct3D headers etc.

tested wherever relevant.

sub-millisecond but can be improved if we get rid of SDL timers.