glretrace, dump-images: Accept --call-nos=no to get snapshots with call numbers
It is sometimes inconvenient to have call numbers encoded within the
snapshot filenames. For example, when doing tests of "apitrace trim"
in the test suite the call numbers frequently change even when the
image contents do not.
By accepting the --call-nos=no option, these programs will now
generate filenames with sequential numbers starting from 0 rather than
using call numbers in the filenames.