Support topologies other than TriangleList (#3349)

# Objective

Fixes https://github.com/bevyengine/bevy/issues/3346

## Solution

I've encoded the topology in the `MeshKey` similar to how MSAA samples are handled.

crates/bevy_pbr/src/render/mesh.rs

@@ -370,12 +370,15 @@ bitflags::bitflags! {
         const VERTEX_TANGENTS             = (1 << 0);
         const TRANSPARENT_MAIN_PASS       = (1 << 1);
         const MSAA_RESERVED_BITS          = MeshPipelineKey::MSAA_MASK_BITS << MeshPipelineKey::MSAA_SHIFT_BITS;
+        const PRIMITIVE_TOPOLOGY_RESERVED_BITS = MeshPipelineKey::PRIMITIVE_TOPOLOGY_MASK_BITS << MeshPipelineKey::PRIMITIVE_TOPOLOGY_SHIFT_BITS;
     }
 }
 
 impl MeshPipelineKey {
     const MSAA_MASK_BITS: u32 = 0b111111;
     const MSAA_SHIFT_BITS: u32 = 32 - 6;
+    const PRIMITIVE_TOPOLOGY_MASK_BITS: u32 = 0b111;
+    const PRIMITIVE_TOPOLOGY_SHIFT_BITS: u32 = Self::MSAA_SHIFT_BITS - 3;
 
     pub fn from_msaa_samples(msaa_samples: u32) -> Self {
         let msaa_bits = ((msaa_samples - 1) & Self::MSAA_MASK_BITS) << Self::MSAA_SHIFT_BITS;
@@ -385,6 +388,26 @@ impl MeshPipelineKey {
     pub fn msaa_samples(&self) -> u32 {
         ((self.bits >> Self::MSAA_SHIFT_BITS) & Self::MSAA_MASK_BITS) + 1
     }
+
+    pub fn from_primitive_topology(primitive_topology: PrimitiveTopology) -> Self {
+        let primitive_topology_bits = ((primitive_topology as u32)
+            & Self::PRIMITIVE_TOPOLOGY_MASK_BITS)
+            << Self::PRIMITIVE_TOPOLOGY_SHIFT_BITS;
+        MeshPipelineKey::from_bits(primitive_topology_bits).unwrap()
+    }
+
+    pub fn primitive_topology(&self) -> PrimitiveTopology {
+        let primitive_topology_bits =
+            (self.bits >> Self::PRIMITIVE_TOPOLOGY_SHIFT_BITS) & Self::PRIMITIVE_TOPOLOGY_MASK_BITS;
+        match primitive_topology_bits {
+            x if x == PrimitiveTopology::PointList as u32 => PrimitiveTopology::PointList,
+            x if x == PrimitiveTopology::LineList as u32 => PrimitiveTopology::LineList,
+            x if x == PrimitiveTopology::LineStrip as u32 => PrimitiveTopology::LineStrip,
+            x if x == PrimitiveTopology::TriangleList as u32 => PrimitiveTopology::TriangleList,
+            x if x == PrimitiveTopology::TriangleStrip as u32 => PrimitiveTopology::TriangleStrip,
+            _ => PrimitiveTopology::default(),
+        }
+    }
 }
 
 impl SpecializedPipeline for MeshPipeline {
@@ -496,7 +519,7 @@ impl SpecializedPipeline for MeshPipeline {
                 polygon_mode: PolygonMode::Fill,
                 clamp_depth: false,
                 conservative: false,
-                topology: PrimitiveTopology::TriangleList,
+                topology: key.primitive_topology(),
                 strip_index_format: None,
             },
             depth_stencil: Some(DepthStencilState {

crates/bevy_pbr/src/render/mod.rs

@@ -269,6 +269,8 @@ pub fn queue_meshes(
                         if mesh.has_tangents {
                             pbr_key.mesh_key |= MeshPipelineKey::VERTEX_TANGENTS;
                         }
+                        pbr_key.mesh_key |=
+                            MeshPipelineKey::from_primitive_topology(mesh.primitive_topology);
                     }
 
                     if let AlphaMode::Blend = material.alpha_mode {

crates/bevy_render/src/mesh/mesh/mod.rs

@@ -593,6 +593,7 @@ pub struct GpuMesh {
     pub vertex_buffer: Buffer,
     pub index_info: Option<GpuIndexInfo>,
     pub has_tangents: bool,
+    pub primitive_topology: PrimitiveTopology,
 }
 
 /// The index info of a [`GpuMesh`].
@@ -640,6 +641,7 @@ impl RenderAsset for Mesh {
             vertex_buffer,
             index_info,
             has_tangents: mesh.attributes.contains_key(Mesh::ATTRIBUTE_TANGENT),
+            primitive_topology: mesh.primitive_topology(),
         })
     }
 }