This shouldn't be in the hardware independent interface because the semantics on OpenGL and Vulkan are too different, so a common implementation is not possible.
Most bind calls were in the GL interface anyway, so these no longer pass through hardware independent code.
This also moves the bind calls in the shadowmap code into the GL interface - these never did anything useful in Vulkan and aren't needed there.
Last but not least, this moves the legacy buffer binding handling into FGLRenderState and performs the initial binding for the light buffer in a more suitable place so that this doesn't have to pollute the render state.
currentUILevel is now primaryLevel.
For ZScript, currentVMLevel was added. This is also exported as 'level' and will change as needed.
This also means that no breaking deprecations will be needed in the future, because in order to sandbox a level only 4 variables need to be handled: level, players, playeringame and consoleplayer.
The remaining global variables are not relevant for the level state.
The static 'level' has been mostly removed from the code except some places that still need work.
