make pbm screen sizes less hardcoded

[matthijskooijman]

The .c files for the pbm bitmap-writing virtual devices contain
constants for the output width and height. Previously these constants
were used throughout the code, making it impossible to change the width
and height without changing these constants.

With this change, only the initialization uses the constants, but the
rest of the code reads back the width and height from the device
struct and uses those. This enables a user of these devices to change
the width and height to change the size of the output.

This was tested for all three of the pbm devices.