As some Mesa developers pointed out, the GL_ARB_vertex_program grammar
does not allow an immediate value as source argument in ARL. Most
compilers accept it, but since it is not the purpose of the test
program to test for this replace it with a proper constant.

Changing the texture unit when binding a surface for loading can break
the state manager in the way that it changes the currently active
texture unit while it is setting up a texture that has to be loaded.
Instead find out the current unit to dirtify the correct sampler.

Fix RegSetValueA/W to return ERROR_INVALID_PARAMETER if NULL data is 
passed in.