r_refdef is really meant for holding the various screen "constants" for
the software renderer rather than the more generic scene stuff. All the
fields referenced by the low level rendering code (especially assembly)
have been moved to the beginning of the struct (and nicely fit within 64
bytes). The other fields should be moved elsewhere, but not this commit.
On top of that, R_ViewChanged is much easier to read, and there are
fewer static globals.
This moves the common camera setup code out of the individual drivers,
and completely removes vup/vright/vpn from the non-software renderers.
This has highlighted the craziness around AngleVectors with it putting
+X forward, -Y right and +Z up. The main issue with this is it requires
a 90 degree pre-rotation about the Z axis to get the camera pointing in
the right direction, and that's for the native sw renderer (vulkan needs
a 90 degree pre-rotation about X, and gl and glsl need to invert an
axis, too), though at least it's just a matrix swizzle and vector
negation. However, it does mean the camera matrices can't be used
directly.
Also rename vpn to vfwd (still abbreviated, but fwd is much clearer in
meaning (to me, at least) than pn (plane normal, I guess, but which
way?)).
