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/Font.cpp

@@ -92,6 +92,8 @@ Font::Font(love::font::Rasterizer *r, const SamplerState &s)
 	if (pixelFormat == PIXELFORMAT_LA8_UNORM && !gfx->isPixelFormatSupported(pixelFormat, PIXELFORMATUSAGEFLAGS_SAMPLE))
 		pixelFormat = PIXELFORMAT_RGBA8_UNORM;
 
+	vertexAttributesID = gfx->registerVertexAttributes(VertexAttributes(vertexFormat, 0));
+
 	loadVolatile();
 	++fontCount;
 }