DXMT: keep indexed draw base vertex signed

[moreaki]

Summary

Keep indexed-draw base-vertex values signed across the bundled DXMT/winemetal command boundary.

BaseVertexLocation is signed in D3D11, and Metal's indexed-draw baseVertex argument is signed. The two intermediate command fields are currently uint32_t; this patch changes only those fields to int32_t. Their size and command layout remain unchanged.

Deterministic reproduction

Use an indexed draw with BaseVertexLocation = -1.

1. MTLD3D11DeviceContextImpl::DrawIndexed receives the correct signed INT value.
2. Assigning it to DXMT_DRAW_INDEXED_ARGUMENTS::BaseVertex currently stores 0xffffffff in an unsigned field.
3. Assigning that to wmtcmd_render_draw_indexed::base_vertex keeps it unsigned.
4. Objective-C then promotes the field for Metal's signed baseVertex parameter as positive 4294967295, not -1.

Keeping both fields signed preserves -1 through the existing command path.

Validation

• Rebased independently on current origin/main (5ac81eda).
• git diff --check passes.
• Cherry-picked on top of the LLVM 15 build prerequisite from DXMT: fix the supported LLVM 15 build #748 and built kmk VBoxDxMt on macOS/Arm with LLVM 15.0.7.
• Included in the clean integrated macOS/Arm full build, which completed with exit 0 and produced VBoxDxMt.dylib.

Closes #754.

Signed-off-by: Roberto Nibali rnibali@gmail.com

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[moreaki]

DXMT v0.80 audit update:

This is already addressed in DXMT v0.80: the corresponding D3D11 indexed draw command state uses a signed int32_t base-vertex field. Updating the vendored DXMT tree to v0.80 would include the equivalent fix.

So this PR is best understood as a narrow backport for VirtualBox's currently vendored DXMT snapshot.