Fix regression with split-screen multi render in Quake 3. Caused by
aef3d2d337.
Move duplicated code into transform_matrix. Handle framebuffer
dimensions/clipping scaling depending on render mode (Full FB emu, copy
RTT to vram).
Handle RTT and screen render similarly when possible.
[ci skip]
Fix framebuffer clipping when scale>1 (Stretched PAL). Issue #728
Get rid of ScissorMatrix.
Don't scale framebuffer clipping rect in full FB emu.
Refactor rend_context. Use Rect and ivec2.
Nejsplitter