Chart test failures on mac: text size differences

Hi Khaled,

I see you're working on text rendering, so I thought that my findings might be usefull for you. I 
had a look at chart test failures on Mac and found that it is because of text size differences. (I 
checked your latest change [1], but the differences are still there.)
Both text width and height is different (comparing Windows and Mac). Interestingly when I changed 
the font to a bundled, monospace font (e.g. Liberation Mono, DejaVu Sans Mono), width differences 
disappeared and only height differences remained. So there are at least two kind of issues here. 
One which is about the proportional fonts character width and one related to the text height.
I added a minimal test case to the source [2] with which it's easier to debug the second issue (for 
the first issue the test case's font need to be changed to a bundled, proportional font).

[1] 
https://cgit.freedesktop.org/libreoffice/core/commit/?id=04d086531ff4a3ab24b704489270c7c23792bd20
[2] 
https://cgit.freedesktop.org/libreoffice/core/commit/?id=4b8cb3e3b9c597108697f46bcc93045b06cc0633

Best Regards,
Tamás