SDL Mouse Button clean up

Make 'b' variable an int so that key values more than 255 will work (not currently an issue in ioq3 afaik).
Use SDL_BUTTON_* for easier transition to SDL2.
(SDL2 doesn't have SDL_BUTTON_WHEELUP or SDL_BUTTON_WHEELDOWN, X1/X2 are different values.)
Add if .. endif support to shaders

Supports keywords used by FAKK, Alice, and EF2. Unknown keywords are assumed to be cvars. Also support #if and #endif.
Fix "if shaderlod", add r_shaderlod cvar

From my tests "if shaderlod <float-value>" works like it does in Alice now.
Update README: No more com/fs_basegame, add r_shaderlod
Merge remote-tracking branch 'ioquake3/master'

Fixes mouse AUX keys, they're > 255 in spearmint.