GLEW normally backs directly onto glXGetProcAddress on Linux, which is broken
on non-GLX setups, such as Wayland (but also X11 with EGL, and presumably KMSDRM).
Replace it with manual calls to SDL_GL_GetProcAddress() for the VBO path.
Note, however, that SDL_opengl.h includes "windows.h", so conflicts with the
miniwin implementation, which breaks builds on windows.
In order to work around this, we do what the Direct3D9 implementation does and
push all of the OpenGL calls to a separate file, actual.cpp.
Going forward, it may make sense to load _all_ OpenGL entry points via SDL,
which would allow us to avoid linking directly with libGL/libOpenGL, and
therefore eliminate the separate build dependency altogether, as well as
allowing more runtime configurability as to the OpenGL library to load.
(But that's definitely a bit uglier, and also useful very rarely.)
On some systems, the old-style GLEW_TARGET_LIBRARIES variable is not set by
find_package(GLEW), and instead we must use the newer GLEW::GLEW target, as
we do for OpenGL.
This fixes a build error on openSUSE Tumbleweed, which has a glew CMake config
in /usr/lib64/cmake/glew/glew-config.cmake.
