renderers: allocate less poly buffers and fix max checks

opengl1/gles used r_maxpolys for range check, but should of used MAX_POLYS.
opengl2 used r_maxpolyverts, but should of used r_maxpolys.

Instead add MAX_POLYBUFFERS 4096, the same value vanilla ET used for MAX_POLYS.
Vanilla cgame only uses a max of 128 poly buffers.
Allowing 16384 (gl1/gles) or 10000 (gl2) seems like a waste.