`select` filter doesn't support `ih` and `iw` constants

[alexey.rodionov]

Running e.g. the following command:

ffmpeg -i input.mp4 -vf select='gt(ih\,1080)',scale=-2:1080 output.mp4

causes the following error:

[Parsed_select_0 @ 00000196cee7f500] [Eval @ 00000087f25fe3d0]
Undefined constant or missing '(' in 'ih,1080)'
[Parsed_select_0 @ 00000196cee7f500] Error while parsing expression
'gt(ih,1080)'
[AVFilterGraph @ 00000196cee7f440] Error initializing filters
Invalid argument

Support for ih and iw constants in the select filter would allow interesting complex one-line filters.