reenabled loading plaques (and cleaned up that code a lot)

reenabled loading plaques (and cleaned up that code a lot)
moved screenshot/envmap code from gl_backend.c to cl_screen.c (except for the GL SCR_ScreenShot function of course)
cleaned up trailing whitespace in console.c
backend between-batch overhead is now pretty much non-existent (set up state at start of frame, keep it until end of 3D rendering)
default batch size is now 0 triangles (never batch)
index array is not offset for first mesh in batch (which might be the only mesh in the batch)
does not lock arrays until it tries to render the first mesh
moved GL_LockArray/GL_UnlockArray from gl_models.c to gl_backend.c
made GL calls in r_sky.c unnecessary (now uses R_Mesh_ClearDepth)
made every enum in GL_PrintError (used by CHECKGLERROR) optional, for consistency (not that any GL headers would lack most of them)
inlined MYgluPerspective
removed a few unnecessary GL calls in frame setup code
reformatted uses of CHECKGLERROR to be on the same line as the function they are checking (looks cleaner)
increased MAX_TEXTUREUNITS from 4 to 8 (not that there is any code trying to use more than 2 textures anyway)
calling R_RenderView with no worldmodel is no longer fatal (just returns)
R_Mesh_Clear renamed to R_Mesh_Start
R_Mesh_Finish added (ends 3D rendering)
cleaned up some cruft in screen.h
moved gldepthmin/max from vid_ files to gl_backend, and then commented it out anyway (not currently used)

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@1598 d7cf8633-e32d-0410-b094-e92efae38249