set cull mode: None for Mesh2d (#7514)

# Objective

allow negatively-scaled mesh2ds to render correctly by disabling back-face culling. this brings the mesh2d pipeline into line with the sprite pipeline. i don't see any cases where backface-culling would be useful for 2d meshes.

crates/bevy_sprite/src/mesh2d/mesh.rs

@@ -412,7 +412,7 @@ impl SpecializedMeshPipeline for Mesh2dPipeline {
             layout: Some(vec![self.view_layout.clone(), self.mesh_layout.clone()]),
             primitive: PrimitiveState {
                 front_face: FrontFace::Ccw,
-                cull_mode: Some(Face::Back),
+                cull_mode: None,
                 unclipped_depth: false,
                 polygon_mode: PolygonMode::Fill,
                 conservative: false,