Revert "BUILD: Make GLEW mandatory"

This reverts commit 30888e5493538ca35f90b401a55c12c021ba3dab.
fixup! GRAPHICS: Allow guarded use of Graphics::Framebuffer
fixup! GRAPHICS: Use framebuffer on regular OpenGL too
JANITORIAL: Fix style issues
GRAPHICS: Do not use framebuffers in AmigaOS