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

config.h

@@ -84,6 +84,7 @@ struct config {
 
     enum { STARTUP_WINDOWED, STARTUP_MAXIMIZED, STARTUP_FULLSCREEN } startup_mode;
 
+    bool dpi_aware;
     config_font_list_t fonts[4];
 
     struct {