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>
The origin of image.Images is the upper left corner. Convert the
ReadPixels result by flipping the image the y-direction.
Signed-off-by: Elias Naur <mail@eliasnaur.com>