Document new OpenGL feature

--HG--
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/trunk%401008

README.MiNT

@@ -75,6 +75,9 @@ SDL_VIDEODRIVER:
 	Set to 'xbios' to force xbios video driver
 	Set to 'gem' to force gem video driver
 
+SDL_VIDEO_GL_DRIVER:
+	Set to filename to load as OpenGL library, if you use SDL_GL_LoadLibrary()
+
 SDL_AUDIODRIVER:
 	Set to 'mint_gsxb' to force Atari GSXB audio driver
 	Set to 'mint_mcsn' to force Atari MCSN audio driver
@@ -126,6 +129,21 @@ SDL_JOYSTICK_ATARI:
 ==============================================================================
 VI.  More informations about drivers:
 
+OpenGL:
+	The default is to use the Mesa offscreen driver (osmesa.ldg). If you want
+	to use an older OpenGL implementation, like mesa_gl.ldg or tiny_gl.ldg,
+	your program must use SDL_GL_LoadLibrary() to do so, and retrieve the
+	needed function pointers with SDL_LoadFunction(). In all cases, the OpenGL
+	context is taken care of by SDL itself, you just have to use gl* functions.
+
+	However, there is one OpenGL call that has a different prototype in the old
+	implementations: glOrtho(). In the old implementations, it has 6 float as
+	parameters, in the standard one, it has 6 double parameters. If you want
+	to compile testdyngl, or any other SDL program that loads its OpenGL
+	library, you must change the glOrtho() prototype used in this program. In
+	osmesa.ldg, you can retrieve a glOrtho() with double parameters, by searching
+	for the function "glOrtho6d".
+
 Xbios video:
 	Video chip is detected using the _VDO cookie.
 	Screen enhancers are not supported, but could be if you know how to