In OpenGL, texture coordinates can have up to 4 values. This change
will help with easy application of texture coordinate matrix
transformations in the future.
Additionally, correct the initial value for texture coordinates to
`{ 0.f, 0.f, 0.f, 1.f}`.
Previously we multiplied the interpolated texture coordinates by
width - 1 and height - 1 instead of width and height which resulted in
some wrongly mapped textures, especially visible in the glquake light
maps.
This also corrects the wrap mode being wrongly swapped for s/t
coordinates.
Since we do not have texture borders implemented yet we always use
GL_CLAMP_TO_EDGE for all clamping wrap modes for the time being.
This extracts the sampler functionality into its own class.
Beginning with OpenGL 3 samplers are actual objects, separate
from textures. It makes sense to do this already as it also
cleans up code organization quite a bit.
