RFR: 8369348: Failed assertion with scissor rect when Metal API Validation is enabled

[Ambarish Rapte]

Issue:
This is a metal pipeline specific issue. When Metal API Validation is enabled an assertion is thrown due to invalid scissor rect being set. Please test the steps described in the ticket [JDK-8369348](https://bugs.openjdk.org/browse/JDK-8369348).

Cause:
MetalContext.setClipRect() method uses the negative x,y values and later performs a negative value check.
This results in a clip rect with a width and height being larger than rtt size.

Fix:
Moving the negative value check before first use of x and y variables.
Test is not included as, it requires manually enabling Metal API Validation i.e. METAL_DEVICE_WRAPPER_TYPE

Verification:
Verify the issue described in the [JDK-8369348](https://bugs.openjdk.org/browse/JDK-8369348).
Perform general sanity test.

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Commit messages:
 - check x,y for -ve value before use

Changes: https://git.openjdk.org/jfx/pull/2078/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=2078&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8369348
  Stats: 4 lines in 1 file changed: 2 ins; 2 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/2078.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/2078/head:pull/2078

PR: https://git.openjdk.org/jfx/pull/2078