use location numbers for vertex input attributes, instead of name-based binding.

- add 'location' named field to buffer and mesh vertex format tables, replaces 'name' field.
- location numbers are expected to match 'layout (location = #)' qualifiers in vertex inputs in shader code.
- deprecate vertex inputs in shader code that don't have layout location qualifiers.
- love's default vertex attributes use location 0 for position, 1 for texcoord, and 2 for color.
- add new variants of Mesh:attachAttribute, setAttributeEnabled, and isAttributeEnabled which take location numbers instead of names.

Improves draw performance in vulkan and metal backends.

src/modules/graphics/metal/Shader.h

@@ -61,8 +61,8 @@ public:
 
 	struct RenderPipelineKey
 	{
-		VertexAttributes vertexAttributes;
-		BlendState blend;
+		VertexAttributesID vertexAttributesID;
+		uint32 blendStateKey;
 		uint64 colorRenderTargetFormats;
 		uint32 depthStencilFormat;
 		ColorChannelMask colorChannelMask;