[SDL] Dynamic OpenGL lib support for Mac

[Anders F Björklund]

Here's a patch that adds LoadLibrary and GetProcAddress
to the Carbon macintosh driver (for Mac OS 9 and Mac OS X):
http://www.algonet.se/~afb/SDL-1.2.6-macdynamicgl.patch
It just calls the corresponding function from SDL_loadso.

It also fixes one Mac bug in SDL_loadso.c, that made it fail
always when loading a library, and fixes the screen update
after receiving an update event - which caused the OpenGL
context to be overwritten by a blank window by UpdateRect...

It has no impact on other platforms, nor on the Quartz driver.
--anders

PS. Checked against SDL-1.2.6 and SDL12-CVS without problems.