DXMT: forward D3D10 start instance location

[moreaki]

Summary

Forward the D3D10 DrawIndexedInstanced start-instance argument to the underlying D3D11 context.

The patch changes one argument at the wrapper boundary: the final D3D11 parameter now uses the incoming StartInstanceLocation rather than repeating StartIndexLocation.

Deterministic reproduction

Call the D3D10 wrapper with distinct values, for example:

StartIndexLocation    = 6
StartInstanceLocation = 2

Current code calls the D3D11 context with final argument 6; the D3D10 and D3D11 method declarations both specify that the final argument is the start-instance location, so the forwarded value must be 2. The one-line patch makes that mapping exact.

Impact

D3D10 indexed instanced draws with a non-zero start instance can otherwise use the start-index value as the base instance, selecting incorrect instance IDs or per-instance vertex data.

Validation

• Rebased independently on current origin/main (5ac81eda).
• git diff --check passes.
• Applied 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 #760.

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

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[moreaki]

DXMT v0.80 audit update:

Updating the vendored DXMT tree to v0.80 would not address this. DXMT v0.80 and current DXMT main still pass StartIndexLocation as the final D3D11 DrawIndexedInstanced argument from the D3D10 wrapper.

I opened the equivalent upstream DXMT issue and PR:

• D3D10 DrawIndexedInstanced forwards StartIndexLocation as StartInstanceLocation 3Shain/dxmt#188
• fix(d3d10): forward indexed instanced start instance 3Shain/dxmt#189