Graphics: Switch glyph atlases to R8 instead of A8

A8 can't be supported efficiently on API lacking texture swizzle support (DX, WebGL, WebGPU), so we swizzle in the shader instead

src/Nazara/Graphics/GuillotineTextureAtlas.cpp

@@ -39,7 +39,7 @@ namespace Nz
 		TextureInfo textureInfo;
 		textureInfo.width = size.x;
 		textureInfo.height = size.y;
-		textureInfo.pixelFormat = PixelFormat::A8;
+		textureInfo.pixelFormat = PixelFormat::R8;
 		textureInfo.type = ImageType::E2D;
 		textureInfo.usageFlags = TextureUsage::ShaderSampling | TextureUsage::TransferSource | TextureUsage::TransferDestination;
 		

src/Nazara/Graphics/Resources/Shaders/BasicMaterial.nzsl

@@ -80,7 +80,7 @@ fn main(input: FragIn) -> FragOut
 	let color = settings.BaseColor;
 
 	const if (HasUV)
-		color *= TextureOverlay.Sample(input.uv).rrrg;
+		color.a *= TextureOverlay.Sample(input.uv).r;
 
 	const if (HasColor)
 		color *= input.color;

src/Nazara/Graphics/Resources/Shaders/PhongMaterial.nzsl

@@ -129,7 +129,7 @@ fn main(input: VertToFrag) -> FragOut
 	let color = settings.BaseColor;
 
 	const if (HasUV)
-		color *= TextureOverlay.Sample(input.uv).rrrg;
+		color.a *= TextureOverlay.Sample(input.uv).r;
 
 	const if (HasColor)
 		color *= input.color;

src/Nazara/Graphics/Resources/Shaders/PhysicallyBasedMaterial.nzsl

@@ -115,7 +115,7 @@ fn main(input: VertToFrag) -> FragOut
 	let color = settings.BaseColor;
 
 	const if (HasUV)
-		color *= TextureOverlay.Sample(input.uv).rrrg;
+		color.a *= TextureOverlay.Sample(input.uv).r;
 
 	const if (HasColor)
 		color *= input.color;