Blurry splash screen and plot watermark with display scaling (e.g. 125%) #33

With a display scaling greater than 100% (e.g. 125%), the splash screen and the watermark of empty plots are blurry (see DataLab-Platform/DataLab#460): their image is loaded as a pixmap rendered for a 100% scaling, which Qt upscales by the scaling factor. The fallback splash screen, drawn when no image is available, is also drawn at a 100% scaling.

Proposed fix

• Load the splash screen and watermark images with guidata.configtools.get_pixmap (see Text not scaled with Windows display scaling (Qt 5) and no high-DPI image loading PlotPyStack/guidata#121), which uses their @2x high-resolution variant (e.g. splash@2x.png next to splash.png), if any, to display them at the resolution of the screen.
• Draw the fallback splash screen at the resolution of the screen.