This commit includes the following changes:
* When vid_fullscreen is 1, the game is now drawn centered in the
screen instead of the top left corner, by modifying the viewport and
scissor when drawing the world warp and UI render passes.
* When vid_fullscreen is 1, the world view no longer has an incorrect
size and/or aspect ratio. This was due to the world warp and UI
render passes sampling from the whole color buffer instead of the
restricted render area. To fix this, the postprocessing and world warp
shaders now use unnormalized coordinates, corrected with the frame
offset, and sample directly from the appropriate texels.
* The game no longer outputs pixels which have not been written to,
which are usually displayed black but may come out with undefined
colors. For this, some output color attachments are cleared before
drawing the final elements in the frame.
