support expression in aresample

[yue]

aresample should take an expression rather than an integer value.

suppose I want to write a graph that 1) upsample by 4x, 2) apply biquad filters 3)downsample by 4x

currently I have to write something like "aresample=176400, equalizer ..., aresample=44100". with different file formats it's very inconvenient.

a script would work in a few cases. but not all scenarios can be supported by scripts. It would be better if this could be supported by expression.

it would be better if we can write "aresample='rate *4', equalizer ..., aresample= 'rate / 4'".

one just need to change the integer sample rate option definition in af_asample.c to using av_expr_* parsers would do the job.