Improve GetColorTransparent.

[couet]

As pointed in this issue #10299, a new color was defined each time a transparent color (with alpha channel) was requested. This PR fixes that by checking if a similar transparent color exists.

[phsft-bot]

Starting build on ROOT-debian10-i386/cxx14, ROOT-performance-centos8-multicore/default, ROOT-ubuntu16/nortcxxmod, ROOT-ubuntu2004/soversion, mac1015/python3, mac11/cxx17, windows10/cxx14
How to customize builds

[bellenot]

LGTM, after fixing the clang-format report

[phsft-bot]

Starting build on ROOT-debian10-i386/cxx14, ROOT-performance-centos8-multicore/default, ROOT-ubuntu16/nortcxxmod, ROOT-ubuntu2004/soversion, mac1015/python3, mac11/cxx17, windows10/cxx14
How to customize builds

[phsft-bot]

Build failed on ROOT-debian10-i386/cxx14.
Running on pcepsft11.dyndns.cern.ch:/home/sftnight/build/workspace/root-pullrequests-build
See console output.

Errors:

• [2022-04-04T13:25:17.174Z] stderr: error: Failed to merge in the changes.
• [2022-04-04T13:25:22.231Z] CMake Error at /home/sftnight/build/workspace/root-pullrequests-build/rootspi/jenkins/root-build.cmake:1083 (message):

[phsft-bot]

Build failed on ROOT-performance-centos8-multicore/default.
Running on olbdw-01.cern.ch:/data/sftnight/workspace/root-pullrequests-build
See console output.

Errors:

• [2022-04-04T15:07:18.411Z] CMake Error at /data/sftnight/workspace/root-pullrequests-build/rootspi/jenkins/root-build.cmake:1083 (message):

[phsft-bot]

Build failed on ROOT-ubuntu2004/soversion.
Running on root-ubuntu-2004-1.cern.ch:/home/sftnight/build/workspace/root-pullrequests-build
See console output.

Errors:

• [2022-04-04T15:23:07.876Z] stderr: error: Failed to merge in the changes.
• [2022-04-04T15:23:13.026Z] CMake Error at /home/sftnight/build/workspace/root-pullrequests-build/rootspi/jenkins/root-build.cmake:1083 (message):

[phsft-bot]

Build failed on windows10/cxx14.
Running on null:C:\build\workspace\root-pullrequests-build
See console output.

Failing tests:

• projectroot.roottest.python.cmdLineUtils.roottest_python_cmdLineUtils_NameCyclesRootmvPrepareInput
• projectroot.roottest.python.cmdLineUtils.roottest_python_cmdLineUtils_NameCyclesRootmv
• projectroot.roottest.python.cmdLineUtils.roottest_python_cmdLineUtils_NameCyclesRootmvCheckOutput
• projectroot.roottest.python.ttree.roottest_python_ttree_ttree
• projectroot.roottest.root.dataframe.roottest_root_dataframe_test_chainZombieFile_WILL_FAIL
• projectroot.roottest.root.dataframe.roottest_root_dataframe_missingBranches_WILL_FAIL
• projectroot.roottest.root.dataframe.roottest_root_dataframe_test_snapshotNFiles
• projectroot.roottest.root.dataframe.roottest_root_dataframe_test_listFilesCtor
• projectroot.roottest.root.dataframe.roottest_root_dataframe_testIMT
• projectroot.roottest.root.dataframe.roottest_root_dataframe_test_readTotemNtuple

And 60 more

[phsft-bot]

Build failed on mac1015/python3.
Running on macitois22.dyndns.cern.ch:/Users/sftnight/build/workspace/root-pullrequests-build
See console output.

Errors:

• [2022-04-04T18:55:46.572Z] stderr: error: Failed to merge in the changes.
• [2022-04-04T18:55:52.273Z] CMake Error at /Volumes/HD2/build/workspace/root-pullrequests-build/rootspi/jenkins/root-build.cmake:1083 (message):

[phsft-bot]

Build failed on mac11/cxx17.
Running on macphsft20.dyndns.cern.ch:/Users/sftnight/build/workspace/root-pullrequests-build
See console output.

Errors:

• [2022-04-04T21:01:51.521Z] CMake Error at /Users/sftnight/build/workspace/root-pullrequests-build/rootspi/jenkins/root-build.cmake:1083 (message):