vcl: Rename GetImplFontCapabilities()

The data structure is not ImplFontCapabilities but FontCapabilities,
therefore the function name is very misleading.

Change-Id: I90f074902af4b4bee79cd445036dc24797836ac1
Reviewed-on: https://gerrit.libreoffice.org/11454
Reviewed-by: Chris Sherlock <chris.sherlock79@gmail.com>
Tested-by: Chris Sherlock <chris.sherlock79@gmail.com>