#9130 Text in main work area isn't rendered with high resolution
Introduce the concept of pixel ratio: the ratio of physical and device independent pixels.
This is useful for rendering of content on Retina-displays of Mac hardware with high resolution.
Qt has real support for this starting with Qt5 - therefore it has to be compiled conditionally.
This change uses some work of Marcelo Galvão Póvoa, thank you.
#9130 Text in main work area isn't rendered with high resolution
Add search mode check_hidpi to ease the lookup for images with double size to use for displays with high resolution.
#9130 Text in main work area isn't rendered with high resolution
Add pixel_ratio to graphics params to use it for displays with high resolution.
It holds the ratio between physical pixels and device-independent pixels of the graphics.
 #9130 Text in main work area isn't rendered with high resolution
Add display_pixel_ratio to buffer params to use it for displays with high resolution.
It holds the highest ratio between physical pixels and device-independent pixels of the LyX application.
Preview snippets will be generated using this value to get high resolution preview.
#9130 Text in main work area isn't rendered with high resolution
Assign the pixel_ratio of graphics params to the Qt pixmap of the image.
#9130 Text in main work area isn't rendered with high resolution
Make QT_VERSION check for Qt5 matching Qt5.0.0 too.
#9130 Text in main work area isn't rendered with high resolution
Load images with search mode check_hidpi to read images with double size when present.
#9130 Text in main work area isn't rendered with high resolution
Assign the pixelRatio of the application to the buffer of the work area.
#9130 Text in main work area isn't rendered with high resolution
Add a LyX banner with double resolution for displays with high resolution.