RandR 1.1 has a physical size for each mode. It used to be that the DIX would
remember these modes and pass them back up to the DDX when changing the screen
configuration. The DDX uses RR_GET_MODE_MM to query the driver for the physical
dimensions of the screen, allowing it to preserve the DPI.
With RandR 1.2, the physical dimensions are stored as part of the output, rather
than per mode. The DIX only uses the sizes passed in from the DDX to select the
mode pool for the "default" output, and forgets the physical sizes. Then, when
reconfiguring the screen, it makes up a new RRScreenSizeRec using the dimensions
from the output, screwing up the DPI.
This change works around this problem by ignoring the DIX and querying the real
size from the driver.
(cherry picked from commit 2e8daee053)
75b5e999f0