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/renderercommon/qgl.h

@@ -305,4 +305,7 @@ QGL_ARB_vertex_array_object_PROCS;
 QGL_EXT_direct_state_access_PROCS;
 #undef GLE
 
+extern int qglMajorVersion, qglMinorVersion;
+#define QGL_VERSION_ATLEAST( major, minor ) ( qglMajorVersion > major || ( qglMajorVersion == major && qglMinorVersion >= minor ) )
+
 #endif