Rotate text from the centroid of the bounding box

[jacebrowning]

This will match how overlay images are rotated:

memegen/app/utils/images.py

Lines 188 to 201 in a678738

	# This offset math is inexact, but works well enough to see
	# approximately where rotated overlay images will be placed.
	# TODO: implement a proper solution using trigonometry.
	angle = abs(overlay.angle)
	if angle > 45:
	angle = 90 - angle
	offset = (
	int(angle + (22.5 - angle) / 22.5 - 1),
	int(angle + (22.5 - angle) / 22.5 - 1),
	)
	x1, y1, _x2, _y2 = overlay.get_box(image.size)
	point = (x1 - offset[0], y1 - offset[1])
	box = box.rotate(overlay.angle, expand=True)