DPI/Size calculation should be done relative to the "current" monitor

Submitted by Carlos Garnacho @carlosg

Link to original bug (#753675)

Description

I recently filed bug #753621 to gtk+ fixing gdk_screen_get_width/height_mm() on Wayland. This fixed evince document size calculations, as it all indirectly relies on ev_document_misc_get_screen_dpi().

Nonetheless, it could be worth to adapt this to be per-monitor, as it was mentioned in that bug, the GdkScreen API doesn't look so viable in the future, plus it looks like it could help on mixed regular/hidpi scenarios.