vo_gl3: make it work on OpenGL 2.1

Now vo_gl3 should work with standard OpenGL 2.1, as long as the
GL_ARB_texture_rg extension is available. Optional features, which
require features that are always in OpenGL 3.0, but are available
as extensions only in OpenGL 2.1, are automatically disabled.

The force-gl2 suboption, which was an unreliable hack to run vo_gl3
in an OpenGL 2.1 context, is removed.

Significant changes are done to the extension loader to make it easier
to identify optional OpenGL features.

Context creation is a bit changed to simplify the code and to handle
the fallback better if OpenGL 3 context creation fails, and creating
an OpenGL legacy context is attempted.

Based on the initial work by Rudolf Polzer <divverent@xonotic.org>,
which included making the shader GLSL 1.20 compatible, and more.

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