Add configuration of Hi-DPI canvas scaling in OpenGL
First, add automatic detection of toolkit DPI scaling options. For now, this
is, in order,
* Check the GDK_SCALE option under GTK+ (users can set this to force the scaling)
* Check the value from WX's GetContentScalingFactor(). This will start to work
correctly from WX 3.1 and GTK+ 3.10.
Then, add a user-settable override in the main prefs panel, next to the icon
scaling. This is independent of the icon scaling options.
DPI handling is performed in a standalone class, so they can be shared between
the prefs UI and the OpenGL backend easily. Also means Cairo could use the same
interface in future.
Also adjust the OpenGL grid drawing code to use the computed scale factor,
which avoids over-thick grids in scaled environments (the user can manually
thicken the grid if wanted).
Gerbview: fix crash when reading a .gbrjob, and when a file given by the .gbrjob is not found.
A incorrect gerber image was created, creating crash on exit or when enable the layer view.
Fix incorrect detection of filled zones changes in DRC check.
Commit 6006703798 fixed a crash but broke the filled zones changes detection.
Filled zones were always seen as not up to date due to the fact the filled areas were cleared too early.
The up to date detection is also optimized: the old filled polygons are no longer stored.
Instead of, the MD5_HASH is calculated and stored before clearing the filled polygons.
Apparently curly brace escaping in a list does not work with Doxygen's
markdown support which caused everything after the windows install path
to be removed from the output build. I just fell back to the windows
environment variable expansion character (%) and that resolved the issue.