Cleaned up WGL_ACCELERATION_ARB usage.

We now do FULL or NO accel based on the app's preference. If the app didn't
 specify, we do FULL then fall back to NO.

(Not specifying anything--a true "don't care" scenario--breaks some ATI
 drivers, so we try to keep to the spirit of it while forcing a specific
 state.)

Previously, it would always do FULL, and try NO if it failed and the app
 had requested NO or DONTCARE.

This is a transplant of hg changesets a04171d6fa11 and d0b7c45e982e from the
 SDL-1.2 branch.

Fixes Bugzilla #1254.

--HG--
extra : rebase_source : db951d96e685e17a4d71fe2aa3d65043661ccccc
Fixed compiler warning.
Removed some unused variables.
Fixed compiler warnings on Haiku.