This makes demos render correctly on hiDPI displays,
while also demonstrating how to handle multiple resolutions.
The 3D in 2D demo now uses "3D No-Effects" for the 3D viewport,
which is faster to render. Thanks to this, 4× MSAA is now enabled
for a better result.
The background loading demo now uses mipmaps for better-looking images.
The material testers demo now samples mouse input in a
resolution-independent manner when panning.
Default clear colors were also changed in some projects for visual
consistency with the project's theme.
Opened them, saved all scenes, ran the project. Did necessary fixes here and there,
fixed some warnings (but some remain in bigger demos).
Did not port JRPG demo yet (#306) nor the two platformer demos.
