Change CullMode to none for sprites (bevyengine#241)

With `CullMode::Back`, a sprite image that is rotated in x,y plane won't display properly

Co-authored-by: kaflu <kaflu@users.noreply.github.com>

Author: kaflu

crates/bevy_sprite/src/render/mod.rs

@@ -22,7 +22,7 @@ pub fn build_sprite_sheet_pipeline(shaders: &mut Assets<Shader>) -> PipelineDesc
     PipelineDescriptor {
         rasterization_state: Some(RasterizationStateDescriptor {
             front_face: FrontFace::Ccw,
-            cull_mode: CullMode::Back,
+            cull_mode: CullMode::None,
             depth_bias: 0,
             depth_bias_slope_scale: 0.0,
             depth_bias_clamp: 0.0,
@@ -70,7 +70,7 @@ pub fn build_sprite_pipeline(shaders: &mut Assets<Shader>) -> PipelineDescriptor
     PipelineDescriptor {
         rasterization_state: Some(RasterizationStateDescriptor {
             front_face: FrontFace::Ccw,
-            cull_mode: CullMode::Back,
+            cull_mode: CullMode::None,
             depth_bias: 0,
             depth_bias_slope_scale: 0.0,
             depth_bias_clamp: 0.0,