Fix .dds file loading in the rendererGL.

The .dds loader was broken on 64 bit systems, because it used outdated
DDS structs. Also the ad-hoc implementation of texture uploads within the
image loader has been moved to tr_image.cpp, where it belongs.

The DXT1 to RGBA texture decompression has been removed, I think the last
desktop GPU that didn't support DXT1 was the Voodoo3.
Add .crn file unpacker.
NormalMap BC5 compression support.

If the hardware doesn't support BC5, the data is converted to BC3 at a
small loss of precision.
Add KTX file loader and cvar to export textures in the GL internal format.
Protect the GL_RGB565 constant.

It is defined only in GL4.1 and later.
Add cubemap loaders for formats that support them natively (crn, ktx)
Enable S3TC on Mesa even if libtxc-dxtn is not available
Merge branch 'texturecompression' into for-0.24