term: stash last known DPI, and use after a unmapped/mapped sequence

A compositor may unmap, and then remap the window, for example when
the window is minimized, or if the user switches workspace.

With DPI aware rendering, we *need* to know on which output we're
mapped, in order to use the correct DPI. This means the first frame we
render, before being mapped, always guesses the DPI.

In an unmap/map sequence, guessing the wrong DPI means the window will
flicker.

Fix by stashing the last used DPI value, and use that instead of
guessing.

This means the *only* time we _actually_ guess the DPI, is the very
first frame, when starting up foot.

terminal.h

@@ -406,6 +406,7 @@ struct terminal {
     struct config_font *font_sizes[4];
     struct pt_or_px font_line_height;
     float font_dpi;
+    float font_dpi_before_unmap;
     bool font_is_sized_by_dpi;
     int16_t font_x_ofs;
     int16_t font_y_ofs;