RFR: 8377427: Reduce substring allocations in Color.web(String, double) [v6]

[Michael Strauß]

> Color.web(string, double) parses a color string by creating substrings of the input. These string allocations can be removed.
> 
> There are no new tests for the `Color` class, since the existing tests already cover all relevant code paths.

Michael Strauß has updated the pull request incrementally with two additional commits since the last revision:

 - review comment
 - review comment

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Changes:
  - all: https://git.openjdk.org/jfx/pull/2069/files
  - new: https://git.openjdk.org/jfx/pull/2069/files/c01400a1..ec29ee00

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=2069&range=05
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=2069&range=04-05

  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jfx/pull/2069.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/2069/head:pull/2069

PR: https://git.openjdk.org/jfx/pull/2069