Only allow model meshes to have SHADER_MAX_VERTEXES - 1 vertexes

The last index is used to check for buffer overflows. See RB_CheckOverflow and RB_EndSurface.
Fix IQM tess buffer overflow

Set the variables that use tess.numVertexes after calling RB_CHECKOVERFLOW() as it may set tess.numVertexes to 0!

Could cause visual issues and error "RB_EndSurface() - SHADER_MAX_VERTEXES hit".