Add RGBA support to Caret

[pushfoo]

tl;dr the rest of the codebase accepts RGBA, but Caret hasn't been updated yet

Other areas of the codebase (shapes, gui, the rest of text) have been updated to accept Tuple[int, int, int, int] for colors. However, Caret has not:

pyglet/pyglet/text/caret.py

Lines 70 to 91 in 5bc0f80

	def __init__(self, layout, batch=None, color=(0, 0, 0)):
	"""Create a caret for a layout.
	By default the layout's batch is used, so the caret does not need to
	be drawn explicitly.
	:Parameters:
	`layout` : `~pyglet.text.layout.TextLayout`
	Layout to control.
	`batch` : `~pyglet.graphics.Batch`
	Graphics batch to add vertices to.
	`color` : (int, int, int)
	RGB tuple with components in range [0, 255].
	"""
	from pyglet import gl
	self._layout = layout
	batch = batch or layout.batch
	group = layout.foreground_decoration_group
	colors = (*color, 255, *color, 255)
	self._list = group.program.vertex_list(2, gl.GL_LINES, batch, group, colors=('Bn', colors))

If there's no technical reason blocking it, can we add it?

[benmoran56]

Sure, that's valid. How about doing it the same way as Shapes, so we can support both RGB and RGBA, not breaking backwards compatibility?