Sk4px: alphas() and Load[24]Alphas()

alphas() extracts the 4 alphas from an existing Sk4px as another Sk4px.
LoadNAlphas() constructs an Sk4px from N packed alphas.

In both cases, we end up with 4x repeated alphas aligned with their pixels.

alphas()
  A0 R0 G0 B0  A1 R1 G1 B1  A2 R2 G2 B2  A3 R3 G3 B3
  ->
  A0 A0 A0 A0  A1 A1 A1 A1  A2 A2 A2 A2  A3 A3 A3 A3

Load4Alphas()
  A0 A1 A2 A3
  ->
  A0 A0 A0 A0  A1 A1 A1 A1  A2 A2 A2 A2  A3 A3 A3 A3

Load2Alphas()
  A0 A1
  ->
  A0 A0 A0 A0  A1 A1 A1 A1  0 0 0 0  0 0 0 0

This is a 5-10% speedup for AA on Intel, and wash on ARM.
AA is still mostly dominated by the final lerp.

alphas() isn't used yet, but it's similar enough to Load[24]Alphas()
that it was easier to write all at once.

BUG=skia:

Review URL: https://codereview.chromium.org/1138333003

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