Fix water reflection on Windows/Virtualbox/Intel graphics

[gvissers]

Fix a number of issues with the water reflection running the client on a Windows client in Virtualbox:

• The framebuffer texture is interpreted upside down by the VirtualBox driver. This is an issue that has been encountered in other programs as well, notably on Intel integrated graphics chipsets. Fix this by detecting if this is the case by generating a known texture and checking its contents, and flipping the scene vertically if it occurs.
• The OpenGL implementation seems to have trouble with the water shader using a vectored multiply-add-assign operation. Explicitly constructing a new vec2 from separate components seems to solve the issue somehow. Though I am no expert in GLSL, the existing code certainly looks valid, so this seems to point to another driver bug.

Posting this as a pull request, so that people may check if it does not have any unwanted side effects. As far as I can tell, there should be none: the first fix should not affect properly behaving systems, and the second just rewrites the same assignment in separate components. I have tested this also in Linux, which was working correctly before, and have observed no change in behaviour. Nevertheless, better get some other testing in before we break an existing setup.

[pjbroad]

It does not appear break the nice water ripples and reflections for my main machine Linux + nvidia; which is good. I'll check on a few other machines when I have time, including and ancient AMD laptop running Windows which does work OK currently.

[gvissers]

Great, thanks for testing!