RFR: 8358130: [Metal] Merge shader compilation tasks

[Ambarish Rapte]

The `build.gradle` file contains separate tasks for compiling each standalone metal shader file.
The 5 tasks are combined into a single task.

Verification:
1. Verify that build completes successfully, and the 5 **.metal** files are correctly compiled and 5 **.air** files are generated in directory : _modules/javafx.graphics/build/msl_
`ClearRttShaders.air`,  `ComputeKernels.air`,  `PassThroughVS.air`,  `PhongPS.air`,  `PhongVS.air`
3. Basic sanity testing

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Commit messages:
 - merge metal shader compilation tasks

Changes: https://git.openjdk.org/jfx/pull/1984/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1984&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8358130
  Stats: 79 lines in 1 file changed: 0 ins; 64 del; 15 mod
  Patch: https://git.openjdk.org/jfx/pull/1984.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1984/head:pull/1984

PR: https://git.openjdk.org/jfx/pull/1984