Fix Freetype build problems
Allow drawing sun in OpenGL1 renderer

Removed #if 0 ... #endif around RB_DrawSun().
Merged improvements and changes from OpenGL2 renderer.
Merge remote-tracking branch 'ioquake3/master'
Use sun shader name from sky shader (RTCW/ET)

Let shaders set sunshader, fallback to (unused Quake 3) "sun" shader
Moved initializing all external shaders to after loading BSP, so could change their names in future. (mainly thinking of opengl2 sunflare shader)
Sky code improves from ET

Fixes ET maps getting error "SHADER_MAX_VERTEXES hit in FillCloudySkySide()"
Only need to run FillCloudBox once in R_BuildCloudData, which gets called for each stage. Fixes crash in ET's radar map.