Add a series of low level gpu.Backend tests to assure the correct behaviour of
Backends. The immediate use is debugging of the Direct3D port, in the future
for developing new backends.
Signed-off-by: Elias Naur <mail@eliasnaur.com>
The macOS backend uses a desktop OpenGL context, not a OpenGL ES
context. The main difference is that sRGB have to be enabled and
a vertex buffer must be bound.
Do that and fix Window.Screenshot for scenes more complex than a
glClear.
Signed-off-by: Elias Naur <mail@eliasnaur.com>
Elias Naur