void al_iphone_override_screen_scale(float scale)
Introduced in 5.0.0

Original iPhones and iPod Touches had a screen resolution of 320x480 (in Portrait mode). When the iPhone 4 and iPod Touch 4th generation devices came out, they were backwards compatible with all old iPhone apps. This means that they assume a 320x480 screen resolution by default, while they actually have a 640x960 pixel screen (exactly 2x on each dimension). An API was added to allow access to the full (or in fact any fraction of the) resolution of the new devices. This function is normally not needed, as in the case when you want a scale of 2.0 for "retina display" resolution (640x960). In that case you would just call al_create_display with the larger width and height parameters. It is not limited to 2.0 scaling factors however. You can use 1.5 or 0.5 or other values in between, however if it's not an exact multiple of the original iPhone resolution, linear filtering will be applied to the final image.

This function should be called BEFORE calling al_create_display.