Replace GLEW dependency with Glad and set the bar at 2.0. Fixes #229 and #228

[fkaa]

This removes GLEW as a dependency and adds Glad instead. We are also bumping the OpenGL version to 2.0.

This means we are doing a few things differently now:

• We ignore the vbo and gl_vbo options, VBOs are "core" in 2.0.
• We show a message box on startup on version mismatch instead of a simple printf.
• We remove gl_max_texunits. This was used incorrectly. If we want to ever go beyond VA_MAX_IMG (8), look into GL_MAX_COMBINED_TEXTURE_IMAGE_UNITS.

If more extensions are required simply generate a new loader and replace the files. The extensions that Glad (3.3 compat) was generated with are the following:

• GL_ARB_debug_output,
• GL_ARB_framebuffer_object
• GL_ARB_framebuffer_sRGB
• GL_ARB_occlusion_query
• GL_EXT_framebuffer_object
• GL_EXT_framebuffer_sRGB
• GL_KHR_debug

[10se1ucgo]

i love you

[fkaa]

Will merge later today. Heads up.