libswscale: Adds conversions from/to float gray format.

libswscale/input.c

libswscale/output.c

libswscale/ppc/swscale_altivec.c

libswscale/swscale_internal.h

libswscale/swscale_unscaled.c

libswscale/utils.c

libswscale/x86/swscale_template.c

tests/ref/fate/filter-pixdesc-grayf32be

tests/ref/fate/filter-pixdesc-grayf32le

tests/ref/fate/filter-pixfmts-copy

tests/ref/fate/filter-pixfmts-crop

tests/ref/fate/filter-pixfmts-field

tests/ref/fate/filter-pixfmts-fieldorder

tests/ref/fate/filter-pixfmts-hflip

tests/ref/fate/filter-pixfmts-il

tests/ref/fate/filter-pixfmts-null

tests/ref/fate/filter-pixfmts-scale

tests/ref/fate/filter-pixfmts-transpose

tests/ref/fate/filter-pixfmts-vflip