RFR: 8377427: Reduce substring allocations in Color.web(String, double) [v4]

[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 one additional commit since the last revision:

  Review comments

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Changes:
  - all: https://git.openjdk.org/jfx/pull/2069/files
  - new: https://git.openjdk.org/jfx/pull/2069/files/f13a5388..b8dbea90

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=2069&range=03
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=2069&range=02-03

  Stats: 50 lines in 3 files changed: 18 ins; 3 del; 29 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