NativeEngine: route instanced generic attributes landing on TexCoord3..TexCoord15

[bkaradzic-microsoft]

Fixes a silent data-loss bug in the divisor-driven instancing reroute in NativeEngine::Draw.

Problem

When Babylon.js declares a generic (consumer-named) vertex attribute with divisor == 1, the attribute is compiled to an ordinary per-vertex bgfx::Attrib slot, but bgfx can only feed per-instance data through the i_data slots (the top TEXCOORD semantics). NativeEngine therefore reroutes such attributes to the correct i_data slot via a lazily-compiled program variant.

That reroute was guarded by attrib < bgfx::Attrib::TexCoord3, so any generic instanced attribute that landed on TexCoord3 or above was silently skipped. BuildInstanceDataBuffer still packed its data into the instance data buffer, but the shader kept reading the attribute as a per-vertex input — i.e. unrelated vertex bytes.

Sprite instancing assigns cellInfo -> TexCoord3, so instanced sprites read garbage cell info and rendered collapsed / mis-UVed.

Fix

Widen the guard to every real bgfx attribute (attrib < bgfx::Attrib::Count).

The built-in instanced attributes (world0-3, splatIndex0-3, instanceColor) are assigned synthetic locations at/above INSTANCE_DATA_FIRST_LOCATION - 4, which is >= bgfx::Attrib::Count, so they still compare false and remain correctly skipped — they already arrive as instance data.

Evidence

Measured on Win32 x64 D3D11 (Debug) with Babylon.js _features.supportSpriteInstancing forced to true, so the instanced sprite path is actually exercised. Negative control = this commit reverted, everything else identical:

test	without fix	with fix
155 Sprites	4.242% FAIL	1.405% PASS
293 Prepass SSAO + sprites	1.617% pass	0.001% pass
369 Sprites Pixel Perfect	25.532% FAIL	7.612% FAIL (much closer, still over threshold)

Full validation suite is unchanged: ran=295 passed=295 failed=0 (--test-index=0-52,58-719; 53-57 are the pre-existing scissor crash on master).

Notes

• No config.json changes. Un-excluding the sprite/MSDF tests additionally requires the Babylon.js-side supportSpriteInstancing flag; this PR is the Native prerequisite for that work and is a strict improvement on its own.
• No shader cache version bump needed. Instanced program variants bypass the shader cache entirely (ShaderProvider::Get consults the cache only when instancedAttributes is empty), and the non-instanced compilation path is untouched.

[Copilot]

Pull request overview

This PR fixes a silent data-loss issue in NativeEngine::Draw where divisor-driven (per-instance) generic vertex attributes were not rerouted to bgfx i_data slots when their compiled attribute location was TexCoord3..TexCoord15, causing shaders to read incorrect per-vertex data (notably impacting sprite instancing).

Changes:

• Expand the instanced-attribute reroute guard from attrib < bgfx::Attrib::TexCoord3 to attrib < bgfx::Attrib::Count so all real bgfx per-vertex attributes are handled.
• Add detailed in-code documentation clarifying why synthetic instance-data locations (>= bgfx::Attrib::Count) remain excluded and how the packing/reroute mapping works.