UpdateImGuiTexture now handles creating font textures and modifying them.

If an icon is displayed on screen before it downloads, it was displaying a default icon but it would fail to load the actual icon even after it was downloaded. This fixes that.

This lets us reduce the number of USE_RETRO_ACHIEVEMENTS ifdefs in the
code base, reducing visual clutter. In particular, needing an ifdef for
each call to IsHardcodeModeActive was annoying to me. This also reduces
the risk that someone writes code that accidentally fails to compile
with USE_RETRO_ACHIEVEMENTS disabled.

We could cut down on ifdefs even further by making HardcodeWarningWidget
always exist, but that would result in non-trivial code ending up in the
binary even with USE_RETRO_ACHIEVEMENTS disabled, so I'm leaving it out
of this PR. It's not a lot of code though, so I might end up revisiting
it at some point.

Up to four leaderboards are displayed in a window in the bottom right of the screen (vertically above challenge icons, if there are any). As per RetroAchievements standards, the markers only display the current leaderboard values with no further context necessary.

The active challenges, aka the primed achievements, are displayed on screen as a series of icons in the bottom right corner of the screen via OnScreenUI.

This was moved to the ~OnScreenUI() destructor in 0d4537d60f695e50379c1542d6c4b1e643e8a5a0.