EGLFB: Incorrect DPI reported on platforms with unknown screen size

We calculate DPI based on detected display with in millimeters compared with the number of pixels along the width of the screen.

Some screens report their width as "2^31 millimeters" (-1 if read as a signed int, but it is delivered as an unsigned value.) This isn't supposed to happen, but clearly is happening. We try to detect this case and default to 96dpi, but due to a coding error in fbdevScreen.c fail to detect it and report 25 dpi. We get to 25dpi because it is 1 pixel-per-millimeter.