Add common OpenGL version parsing + OpenGL 3 fixes

- Parse OpenGL version in sdl_glimp.c to share with both renderers.
- Add GL_VERSION_ATLEAST(major, minor) macro.
- Get address of glGetStringi if using OpenGL 3.
- Fix glConfig.extensions_string when using GL3 core context in
opengl2 renderer.
- Make opengl1 renderer's gfxinfo support qglGetStringi too.

code/renderergl2/tr_local.h

@@ -1336,9 +1336,6 @@ typedef enum {
 // We can't change glConfig_t without breaking DLL/vms compatibility, so
 // store extensions we have here.
 typedef struct {
-	int openglMajorVersion;
-	int openglMinorVersion;
-
 	qboolean    intelGraphics;
 
 	qboolean	occlusionQuery;