C++ will jovially select the implicit conversion operator, even if it's
complete bogus, such as for unknown-size types or non-destructible
types. Therefore, all such conversions (which incur a copy) must
(unfortunately) be explicit so that non-copyable types continue to work.
Before this change, every repaint required a glyph atlas update, which
took 10-20% of the total painting time according to my testing on
different websites. This change improves that significantly by skipping
atlas updated when all glyphs are already present in the texture.
