This is cleaner than the if statements in the code. Also np2 textures
should in theory support linear filtering, but fglrx doesn't seem to
like it. This needs further investigation. So far we've never used
linear filtering on np2 textures, so there should not be a
regression. Furthermore I think shader support is more important than
filtering, since NP2 textures are mostly used for 1:1 copying to the
screen.

ATI cards prior to the radeon HD series did not have unconditional non
power of two support. So far we've used texture_rectangle for that, or
created a bigger power of two texture with padding. This had the
disadvantage that we had to correct the coordinates, which causes
extreme problems with shaders(doesn't work, pretty much).

Both the MacOS and the fglrx driver have support for
GL_ARB_texture_non_power_of_two, and run it on the hardware as long as
we stay within the texture_rectangle limitations. This allows us to
have conditional non power of two textures with normalized
coordinates. This patch adds an internal extension, and the code
creates a regular GL_TEXTURE_2D texture with NP2 size, but refuses
mipmapping, filtering and texture_rectangle incompatible
operations. This makes np2 textures work with shaders on fglrx and
macos.

The opengl extension mentioned in that code was never finished, and as
far as I know there is no way to make use of tangent data in the d3d
fixed function pipeline as well.

Note that GL_ATI_envmap_bumpmap is not the same as
GL_ATI_fragment_shader. envmap_bumpmap is used together with the
regular opengl ffp pipeline and is not used (other than for
pixelformats) if GL_ATI_fragment_shader is used.

Removes unnecessary and unsafe type assignment, adds a missing VariantClear.

Spotted by Dan Kegel.