swr: move dst_size == 0 handling outside DSP function.
[WIP] swr: rewrite resample_common/linear_float_sse/avx in yasm.

Linear interpolation goes from 63->55 (64bit) or 66->59 (32bit)
cycles/sample, and non-linear goes from 43 (llvm) or 38 (gcc) to
32 (yasm) cycles/sample on 64bit, or from 46 (llvm) or 44 (gcc) to
38 (yasm) cycles/sample on 32bit (all testing on OSX 10.9.2, llvm
5.1 and gcc 4.8/9).

DO NOT MERGE. Linear functions not yet updated. AVX and win64 not
yet re-tested.
measure