We have to use the texture limit of the fragment pipeline in use here,
not the fixed function GL texture limit.

Also adds a simple test for this. In the fixed function and nvrc/nvts
code this already works properly.

There is room for 8 texture coordinates, not just 4.

Since some of those function pointers are direct GL functions the function
prototype needs the WINE_GLAPI calling convention. This makes prevents
drawStridedSlow from crashing with USE_WIN32_OPENGL.