I've voided this out on other sound interfaces than SDL Mixer ones because I'm both not sure whether they need it, and not sure how to make them work with it if they do.
This reverts commit 121fcd8369.
The reason I am reverting this is because the last commit actually fixes the *old* screenshot functionality, as the screen is being drawn back onto the buffer after they're swapped in the "real" size. Meaning the old function actually works perfectly fine now.
Only use glCopyTexImage2D when first creating the screen texture, use glCopyTexSubImage2D anytime after that as it does not define a new texture each time.
Flushing of the screen textures has been implemented for when the screen size changes (so that the screen textures don't stay at a wrong size) and the game is closed, I believe they would leave a memory leak before.
Do we really need to keep it around? If not, I say get rid of it. It seems like useless clutter that is just going to confuse people who are trying to understand the source code.
