Previously we would give up on loading the piece images if one fails.
That was awkward because 2 out of 3 places where `set_piece_set_name()`
is called can't propagate errors, so we'd just crash.
Instead, let's skip any images that fail to load, and then when painting
the preview, skip any missing bitmaps and show a warning message that
some are missing.
Now when selecting a chess set in Games Settings, the names of the sets
are capitalized which looks much neater.
The default set 'stelar7' has been renamed to 'Classic' to reflect its
default vanilla design, as opposed to more stylized sets.
This commit also updates any code references to this set.
Previously, calling `.right()` on a `Gfx::Rect` would return the last
column's coordinate still inside the rectangle, or `left + width - 1`.
This is called 'endpoint inclusive' and does not make a lot of sense for
`Gfx::Rect<float>` where a rectangle of width 5 at position (0, 0) would
return 4 as its right side. This same problem exists for `.bottom()`.
This changes `Gfx::Rect` to be endpoint exclusive, which gives us the
nice property that `width = right - left` and `height = bottom - top`.
It enables us to treat `Gfx::Rect<int>` and `Gfx::Rect<float>` exactly
the same.
All users of `Gfx::Rect` have been updated accordingly.
Draw pieces around 80% of the size of a square, instead of 100%, so that
there is a nice gap around them. This feels more comfy, and makes it
actually possible to read the coordinates while a piece is on their
square.
