This converts all usages of bmpman textures to OpenGL texture arrays.
That means that all frames of an animation are stored in a single OpenGL
texture object and can be accessed at the same time in a single draw
call. This should significantly reduce the overhead of drawing lots of
particles since all particles of a single animation can be drawn with a
single draw call.
This fixes#1169.
Thanks to the new material system, it's now possible to easily make the
2D functions independent of the underlying graphics API which allows to
reduce the size of the gr_screen 2D API.