This patch is a follow-up fix for "ttf_font: prevent out-of-bounds
access", which produced sporadic single-pixel artifacts with large font
sizes, e.g., with the monospaced font set to 24px in Sculpt.
The artifacts occurred only for some horizontal sub-pixel positions, in
combination with the font cache, and after the rendering of '>'
characters. They were ultimately caused by the missed clearning of the
first pixel of a glyph where x0 is 1 (e.g., the character 'd'). In this
case, a pixel from the previously evicted cache entry (the '>') shined
through. The patch fixes the problem by clearing the glyph starting from
the first, not the x0's, pixel.
Fixes#3567
