Guard particle spawn texcoord fetch against missing UVs

The particle spawn compute shader read emitter UVs unconditionally. For a
mesh without UVs, spawnMeshTexcoordsIdx == BINDING_INDEX_INVALID (0xFFFF),
so reading geometries[0xFFFF] is an out-of-range bindless descriptor access
that faults the GPU (VK_ERROR_DEVICE_LOST). Wrap the texcoord fetch and its
use in a BINDING_INDEX_INVALID guard, matching the existing color and
prev-position guards, so emitter meshes lacking UVs no longer crash.

Fixes #1100

Author: AlexDunn

src/dxvk/shaders/rtx/pass/particles/particle_system_spawn.comp.slang

@@ -106,11 +106,19 @@ GpuParticle respawn(uint particleIdx, uint spawnCtxIdx, const GpuParticleSystem
       color += unorm4x8ToFloat4x32(colorBits).zyxw * randomBary[i];
     }
 
-    const uint baseSrcTexcoordOffset = (spawnCtx.spawnMeshTexcoordsOffset + index * spawnCtx.spawnMeshTexcoordsStride) / 4;
-    float2 texcoord = float2(BUFFER_ARRAY(geometries, (uint)spawnCtx.spawnMeshTexcoordsIdx, baseSrcTexcoordOffset + 0),
-                             BUFFER_ARRAY(geometries, (uint)spawnCtx.spawnMeshTexcoordsIdx, baseSrcTexcoordOffset + 1));
+    // Only fetch emitter UVs when a valid texcoord buffer is bound. For a mesh without UVs,
+    // spawnMeshTexcoordsIdx == BINDING_INDEX_INVALID (0xFFFF); reading geometries[0xFFFF] is an
+    // out-of-range bindless descriptor access and faults the GPU (device loss). Mirrors the
+    // existing guards on the color and prev-position reads above.
+    float2 texcoord = float2(0.0f, 0.0f);
+    if (spawnCtx.spawnMeshTexcoordsIdx != BINDING_INDEX_INVALID)
+    {
+      const uint baseSrcTexcoordOffset = (spawnCtx.spawnMeshTexcoordsOffset + index * spawnCtx.spawnMeshTexcoordsStride) / 4;
+      texcoord = float2(BUFFER_ARRAY(geometries, (uint)spawnCtx.spawnMeshTexcoordsIdx, baseSrcTexcoordOffset + 0),
+                        BUFFER_ARRAY(geometries, (uint)spawnCtx.spawnMeshTexcoordsIdx, baseSrcTexcoordOffset + 1));
+    }
 
-    if (particleSystem.desc.useSpawnTexcoords)
+    if (particleSystem.desc.useSpawnTexcoords && spawnCtx.spawnMeshTexcoordsIdx != BINDING_INDEX_INVALID)
     {
       if (i == 0)
       {