Ensure instance_index push constant is always used in prepass.wgsl (#10706)

# Objective

 Kind of helps #10509 

## Solution

Add a line to `prepass.wgsl` that ensure the `instance_index` push
constant is always used on WebGL 2. This is not a full fix, as the
_second_ a custom shader is used that doesn't use the push constant, the
breakage will resurface. We have satisfying medium term and long term
solutions. This is just a short term hack for 0.12.1 that will make more
cases work. See #10509 for more details.

Author: cart

crates/bevy_pbr/src/prepass/prepass.wgsl

@@ -107,6 +107,12 @@ fn vertex(vertex_no_morph: Vertex) -> VertexOutput {
     // See https://github.com/gfx-rs/naga/issues/2416
     out.instance_index = get_instance_index(vertex_no_morph.instance_index);
 #endif
+#ifdef BASE_INSTANCE_WORKAROUND
+    // Hack: this ensures the push constant is always used, which works around this issue:
+    // https://github.com/bevyengine/bevy/issues/10509
+    // This can be removed when wgpu 0.19 is released
+    out.position.x += min(f32(get_instance_index(0u)), 0.0);
+#endif
 
     return out;
 }

crates/bevy_pbr/src/render/mesh.wgsl

@@ -88,6 +88,13 @@ fn vertex(vertex_no_morph: Vertex) -> VertexOutput {
     out.instance_index = get_instance_index(vertex_no_morph.instance_index);
 #endif
 
+#ifdef BASE_INSTANCE_WORKAROUND
+    // Hack: this ensures the push constant is always used, which works around this issue:
+    // https://github.com/bevyengine/bevy/issues/10509
+    // This can be removed when wgpu 0.19 is released
+    out.position.x += min(f32(get_instance_index(0u)), 0.0);
+#endif
+
     return out;
 }