The pic is scaled to fill the specified rect (then clipped to the
screen (effectively)). Done just for the console background for now, but
it will be used for slice-pics as well.
Not implemented for vulkan yet as I'm still thinking about the
descriptor management needed for the instanced rendering.
Making the conback rendering conditional gave an approximately 3% speed
boost to glsl with the GL stub (~12200fps to ~12550fps), for either
conback render method.
This is the beginning of supporting 2d rendering in 3d space. The idea
is that a canvas can be in 2d orthographic space (not attached to any
entity with a 3d transform), or in 3d perspective space (attached to an
entity with a 3d transform, either as a child of the camera, or of some
object in 3d space).
It will replace the current HUD code when it's working.
