Add a way to force a layer to stay at a certain DPI / zoom level

[csk1]

This is a feature request.

An extremely common complaint about QMS base layers is that the labels and symbols shrink when you print a map. See, for example,

• Issue No labels on Google hybrid in map composer  #43
• This GIS Stackexchange question
• Another GIS Stackexchange question
• A third GIS Stackexchange question.

I know the fundamental problem is that the web-based services provided by this plugin are just not designed for printing, but as you can see from that long list, people really, really want to use QMS layers for printing. It's just so much easier to add a QMS base layer, rather than finding (and possibly paying for) a high-resolution, printable one.

The issue is because the print composer preview is at a lower resolution than the export. The plugin gets tiles at a higher zoom level in order to match the print DPI, and the higher-resolution tiles have smaller labels and smaller features.

Currently, the best solution I've seen is to export the entire map at a lower DPI. But then you lose the detail of all the layers in your map, not just the base layer.

It would be great if there was an option to force the base layer to export with the same DPI as shown on the screen (either in the map canvas or the print composer). I'm picturing a setting in the plugin Settings, something like:

Print/Export Mode (choose one):

• Use tiles at the current zoom level when printing or exporting. Labels and feature will stay the same size as shown in the preview. Printed maps may appear low resolution or "pixellated."
• Use tiles that match the print/export DPI setting, even if they don't match the current preview. Labels and features may change size from how they appear in the preview.

@robintw already figured out part of how to implement this (see here), by setting self.useLastZoomForPrint = True.