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

[Michael Strauß]

Color.web(string, double) parses a color string by creating substrings of the input. Almost all of these string allocations can be removed, except for an invocation of `Double.parseDouble(String)`, which doesn't have an overload that accepts a sub-range of the input string.

There are no new tests for this enhancement, since the existing tests already cover all relevant code paths.

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Commit messages:
 - Reduce substring allocations in Color.web(String, double)

Changes: https://git.openjdk.org/jfx/pull/2069/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=2069&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8377427
  Stats: 223 lines in 1 file changed: 30 ins; 4 del; 189 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