Fix segfault in vpFont

[s-trinh]

Before the following code produces a segfault due to a bad computation of the bottom text bb:

std::ostringstream oss;
oss << "Computation time: " << std::fixed << std::setprecision(2) << 10.5168 << " ms/";

vpFont font(20, vpFont::TRUETYPE_FILE);
vpImage<vpRGBa> I(32, 256);
font.drawText(I, oss.str(), vpImagePoint(10, 10), vpColor::red);

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 54.56%. Comparing base (917cd82) to head (96cae06).
Report is 19 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1562      +/-   ##
==========================================
+ Coverage   54.39%   54.56%   +0.17%     
==========================================
  Files         442      442              
  Lines       53468    53469       +1     
==========================================
+ Hits        29083    29177      +94     
+ Misses      24385    24292      -93     

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[fspindle]

@s-trinh Thanks