config: add ‘dpi-aware’ option, defaulting to enabled

When disabled, foot no longers uses outputs’ DPI to scale the
font. Instead, it uses the outputs’ scaling factor.

That is, instead of appending “:dpi=123” to the fontconfig string,
modify the “:pixelsize” or “:size” attribute.

Closes #206

terminal.h

@@ -227,6 +227,7 @@ struct terminal {
     struct fcft_font *fonts[4];
     struct config_font *font_sizes[4];
     float font_dpi;
+    int font_scale;
     enum fcft_subpixel font_subpixel;
 
     bool is_sending_paste_data;