metal: fix textures not being tagged with compute-write flags.

src/modules/graphics/metal/Texture.mm

@@ -78,6 +78,8 @@ Texture::Texture(love::graphics::Graphics *gfxbase, id<MTLDevice> device, const
 		desc.usage |= MTLTextureUsageShaderRead;
 	if (renderTarget)
 		desc.usage |= MTLTextureUsageRenderTarget;
+	if (computeWrite)
+		desc.usage |= MTLTextureUsageShaderWrite;
 
 	texture = [device newTextureWithDescriptor:desc];