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.
This commit adds a new cvar called vk_pixel_size that represents how big
pixels should look in the rendered world in order to simulate lower
screen resolutions. With its default value of 1 everything looks normal,
but with bigger sizes (e.g. 4) the rendered world starts to look
"pixelated" due to pixels appearing bigger.
To implement the effect, the viewport and scissor are modified when
drawing the world so the rendering results cover a smaller area in the
top-left corner of the image.
The post-processing fragment shader is used to scale the image back to
the swapchain size before drawing UI elements on top of it.
The UI is not affected by this change, so the existing UI scaling
options continue to work as before with no changes, adding some
flexibility to the mix.
Related to feature request #588.
