Remove octant_offset_c uniform from ubersdf, reorder/document ubersdf uniforms (#191597)

Followup to https://github.com/flutter/flutter/pull/191235 (Move more
RSE rendering cases to complex_rse.frag).

In that PR, there was some discussion about whether removing the
`octant_offset_c` uniform would have an affect on the registers used
reported by malioc. We decided that we didn't need to remove
`octant_offset_c`.

But from testing out the ubesdf gradient changes
(https://github.com/flutter/flutter/pull/190874) after merging with
https://github.com/flutter/flutter/pull/191235, the Mali-G78 registers
used on Vulkan (flutter/impeller/entity/uber_sdf.frag.vkspv) is still
66.

Removing `octant_offset_c` brings the registers used down by 2, as seen
in the malioc changes in this PR. So this is needed to bring down the
registers used to 64 when the gradient changes are included.

This PR also incorporates the reordering and adding of comments to
uber_sdf's uniforms. This way the upcoming gradient change is scoped to
only adding the new gradient functionality for easier review.

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