r28.A "Tremble in Fear"

Compared to upstream r28 release, this release introduces custom kernel support.

For example, a silly way to implement bilinear using custom kernel support is:

scaled = core.fmtc.resample(src, scale=2, custom=lambda x: max(0, 1-abs(x)), support=1) # bilinear upscale

Added parameters: custom/support, custom_h/support_h, custom_v/support_v. They should behave as the rest of fmtconv parameters (including specifying different custom kernel for horizontal/vertical scaling and/or each plane.)

Also note that fmtconv will apply an additional anti-aliasing low-pass filter for downscales (easy to compensate in your custom kernel though)