Refactor 4f gradients using trait templates

Some 4f gradient housekeeping.

1) replace <DstPtrType, ColorProfile> specialization tuples with an enum covering all dest types
   (L32, S32, F16, F32)

2) group various template helpers into dest trait classes, specialized for each dest type

(2a - conflate current dst_swizzle and scale_for_dest ops into one load op)

R=reed@google.com,mtklein@google.com,herb@google.com
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Review URL: https://codereview.chromium.org/1816883002