Make sure all kiva backends implement AbstractGraphicsContext faithfully

[jwiggins]

Since AbstractGraphicsContext is the published and documented interface for kiva backends, we should make sure that all current backends actually use that interface.

• select_font is not consistently implemented [closed by Normalize some signatures of Kiva methods #691]
• Gradients work very differently in the Quartz backend (they are applied immediately as a fill of the current path)
• A number of backends that could implement gradients, don't (PDF, SVG).
• The PDF, PS, and SVG backends have no CompiledPath. PDF could have one (not sure about the others)
• draw_marker_at_points [details in Implement a backend-agnostic version of draw_marker_at_points #708]
• arc takes a keyword argument cw, not clockwise as it's called in several backends (celiagg, qpainter)
• set_flatness is mostly not implemented. (only Cairo has it)
• set_image_interpolation is mostly not implemented (only kiva.agg has it)
• set_text_drawing_mode is mostly not implemented
• The text position and text matrix is not uniformly implemented
• *_character_spacing is not widely implemented
• All backends should implement all the curve commands consistently: arc, arc_to, quad_curve_to, curve_to. (partial fixes in Correct first point and orientation of Kiva QPainter arcs #970 and Ensure all backends implement arc_to and quad_curve_to #988, see also Arcs not drawn correctly when start angle less than end angle #960, Agg backend doesn't draw line to arc start point #961, QPainter backend doesn't draw arcs correctly #962, Discrepancies in arc_to paths #985)

[corranwebster]

The PDF, PS, and SVG backends have no CompiledPath

All the backends that inherit from basecore2d can use the Agg CompiledPath implementation, but they don't expose it in the module API:

enable/kiva/basecore2d.py

Lines 704 to 743 in dae67fe

	def add_path(self, path):
	"""Draw a compiled path into this gc. Note: if the CTM is
	changed and not restored to the identity in the compiled path,
	the CTM change will continue in this GC."""
	# Local import to avoid a dependency if we can avoid it.
	from kiva import agg
	multi_state = 0 # For multi-element path commands we keep the previous
	x_ctrl1 = 0 # information in these variables.
	y_ctrl1 = 0
	x_ctrl2 = 0
	y_ctrl2 = 0
	for x, y, cmd, flag in path._vertices():
	if cmd == agg.path_cmd_line_to:
	self.line_to(x, y)
	elif cmd == agg.path_cmd_move_to:
	self.move_to(x, y)
	elif cmd == agg.path_cmd_stop:
	self.concat_ctm(path.get_kiva_ctm())
	elif cmd == agg.path_cmd_end_poly:
	self.close_path()
	elif cmd == agg.path_cmd_curve3:
	if multi_state == 0:
	x_ctrl1 = x
	y_ctrl1 = y
	multi_state = 1
	else:
	self.quad_curve_to(x_ctrl1, y_ctrl1, x, y)
	multi_state = 0
	elif cmd == agg.path_cmd_curve4:
	if multi_state == 0:
	x_ctrl1 = x
	y_ctrl1 = y
	multi_state = 1
	elif multi_state == 1:
	x_ctrl2 = x
	y_ctrl2 = y
	multi_state = 2
	elif multi_state == 2:
	self.curve_to(x_ctrl1, y_ctrl1, x_ctrl2, y_ctrl2, x, y)

[jwiggins]

I'm pretty averse to creating new dependencies on the Agg backend. We should consider a different way.

[corranwebster]

This isn't new, just noting that there is a default implementation.

I agree thought: I'd really like a pure python implementation of CompiledPath, together with a proper definition of the expected interface.