OBJ_LOADER_DEMO: Fix copy-paste leftover
OBJ_LOADER_DEMO: Calculate the position directly from the texCoord

Instead of from the pixel coord and display size. The function was just un-doing
what we were calculating in outside the function.
OBJ_LOADER_DEMO: The pixelcoord is already being passed in as positionClip
COMMON: Don't force the user to use MSAA'd textures if they use a specific overload

Check the sample number passed in. Only MSAA if the number is over 1. This flexibility
in the C++ code add a bit of headache in the shader code, since MSAA'd textures use a
different schema to sample from them. I added a few #ifdef guards to handle the cases.
OBJ_LOADER_DEMO: Remove unused constant buffer variables