--[[

	Layers API for C, Terra and LuaJIT ffi use.

	- self-allocating constructors.
	- error checking and reporting, separating fatal from non-fatal errors.
	- validation and clamping of enums, indices, counts, sizes, offsets, etc.
	- state invalidation when changing input values.
	- state checking when accessing computed values.

	- using consecutive enum values for forward ABI compatibility.
	- using enlarged number types for forward ABI compat. and clamping inf/-inf.
	- renaming functions and types to C conventions for C use.
	- binding methods and getters/setters via ffi.metatype for LuaJIT use.

	Design considerations:

	- invalid input should never cause crashes.
	- programming errors should trigger asserts.
	- invalid input should not affect other non-child layers in the tree
	  except min_cw and min_ch which can affect the whole geometry.
	- the order in which layer properties are set is not important in order to
	  avoid call-order dependencies, eg. you don't have to set `background_type`
	  to a gradient type before setting `background_color_stop_color`, and you
	  don't have to set `background_color_stop_count` either (the color stops
	  array is expanded automatically). There are exceeptions to this, eg.
	  setting `background_color_stop_count = 0` after adding some color stops
	  _will_ remove those colors stops.

	Usage:

	- make a layer lib object. make top layers from that and then child layers
	  from top layers, constructing a tree. set properties on the layers.
	- call draw() on the top layer. change any properties,
	  check if `pixels_valid` is false, and issue a repaint if it is.
	- hit test layers.
	- navigate, hit-test, select and edit text with cursors.

]]

See the source code for more info.