The dimensions in the `DWRITE_GLYPH_METRICS` are a bit special:
the `rightSideBearing` is measured from the right edge of the
character boundary, not the left edge.
So, doing (right - left) would usually return a negative or very
small number, which, when casted to a u32 in an unsafe block, would
silently overflow to a very large number and make webrender crash.
Therefore, in order to calculate the metrics correctly, we have
to first calculate the full width of the character, then subtract
the right and left edges, same width the height.
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