freetype: fix first glyph non zero bearing alignment offset

When box xMin is 0, lines starting with non 0 bearing can't be aligned properly due to wrong offset

SRT

01
00:00:01,000 --> 00:00:20,000
（s
oo

values

line0box -> 13 .. 35
line1box -> 0 .. 28
boundingbox -> 0 .. 35

line0 width == 22
boundingbox width == 35
wrongly computed offset.x when centering == (35-22)/2 == 6