Fix several gcc warnings in code/graphics/gropengldraw.cpp

variable ‘clipped’ set but not used [-Wunused-but-set-variable]
variable ‘swapped’ set but not used [-Wunused-but-set-variable]
the address of ‘Distortion_texture’ will always evaluate as ‘true’
[-Waddress]
declaration of ‘vertex’ shadows a global declaration [-Wshadow]

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@10943 387891d4-d844-0410-90c0-e4c51a9137d3