Blending for the MTLPixelFormat.r8Unorm texture

Code Block 
		private func createBrushDescriptor(library: MTLLibrary) -> MTLRenderPipelineDescriptor {
				let vertexDescriptor = MTLVertexDescriptor()
				vertexDescriptor.attributes[0].format = .float2
				vertexDescriptor.attributes[0].offset = 0
				vertexDescriptor.attributes[0].bufferIndex = 0
				vertexDescriptor.layouts[0].stride = 2 * MemoryLayout<simd_float1>.size
				vertexDescriptor.layouts[0].stepFunction = .perVertex
				vertexDescriptor.layouts[0].stepRate = 1
				
				let descriptor = MTLRenderPipelineDescriptor()
				descriptor.vertexFunction = library.makeFunction(name: "brush_vertex_function")
				descriptor.vertexDescriptor = vertexDescriptor
				descriptor.fragmentFunction = library.makeFunction(name: "brush_fragment_function")
				descriptor.colorAttachments[0].pixelFormat = .r8Unorm
				
				descriptor.colorAttachments[0].isBlendingEnabled = true
				descriptor.colorAttachments[0].rgbBlendOperation = .add
				descriptor.colorAttachments[0].sourceRGBBlendFactor = .one
				descriptor.colorAttachments[0].destinationRGBBlendFactor = .one
				
				descriptor.depthAttachmentPixelFormat = .invalid
				
				return descriptor
		}