Eliminate GLEW dependency

GL entrypoint resolution is now done manually. This has a couple
immediate benefits, such as not having to retrieve hundreds of
functions pointers that we'll never use. It's also nice to have
an exact overview of all the entrypoints used by mkxp.

This change allows mkxp to run fine with core contexts, not sure
how relevant that is going to be in the future.

What's noteworthy is that  _all_ entrypoints, even the ones core
in 1.1 and guaranteed to be in every libGL, are resolved
dynamically.
This has the added benefit of not having to link directly against
libGL anymore, which also cleans up the output of `ldd` quite
a bit (SDL2 loads most system deps dynamically at runtime).

GL headers are still required at build time.

src/bitmap.cpp

@@ -655,7 +655,7 @@ Color Bitmap::getPixel(int x, int y) const
 	glState.viewport.pushSet(IntRect(0, 0, width(), height()));
 
 	uint8_t pixel[4];
-	glReadPixels(x, y, 1, 1, GL_RGBA, GL_UNSIGNED_BYTE, &pixel);
+	gl.ReadPixels(x, y, 1, 1, GL_RGBA, GL_UNSIGNED_BYTE, &pixel);
 
 	glState.viewport.pop();