The VSE only uses an overlay buffer to draw to the screen. The
GPUViewport assumes that platforms clears textures during creation, but
they do not on selected platforms. What would lead to drawing from
uncleared memory.

By selecting which buffers would contain correct data allows us to use
dummy textures in these cases that would eliminate the drawing
artifacts.

I chose for having a default_buffers parameter in the viewport otherwise
this fix would have code changes in window manager what should not be
aware of how viewports are being used.