TextureFormat::R16Unorm support for Image (bevyengine#5249)

# Objective

Currently some TextureFormats are not supported by the Image type.
The `TextureFormat::R16Unorm` format is useful for storing heightmaps.
This small change would unblock releasing my terrain plugin on bevy 0.8.
 
## Solution

Added `TextureFormat::R16Unorm` support to Image.
This is an alternative (short term solution) to the large texture format issue bevyengine#4124.

crates/bevy_render/src/texture/image.rs

@@ -519,6 +519,7 @@ impl TextureFormatPixelInfo for TextureFormat {
             | TextureFormat::R16Float
             | TextureFormat::Rg16Uint
             | TextureFormat::Rg16Sint
+            | TextureFormat::R16Unorm
             | TextureFormat::Rg16Float
             | TextureFormat::Rgba16Uint
             | TextureFormat::Rgba16Sint
@@ -552,6 +553,7 @@ impl TextureFormatPixelInfo for TextureFormat {
             | TextureFormat::R8Sint
             | TextureFormat::R16Uint
             | TextureFormat::R16Sint
+            | TextureFormat::R16Unorm
             | TextureFormat::R16Float
             | TextureFormat::R32Uint
             | TextureFormat::R32Sint