rect: round the corners named in round_corners - #2098

FPDF.rect() rounds the opposite corner of the one named in round_corners, left and right being swapped:

pdf.rect(50, 50, 100, 100, round_corners=("TOP_LEFT",), corner_radius=20)      # rounds the top right corner
pdf.rect(50, 50, 100, 100, round_corners=("BOTTOM_RIGHT",), corner_radius=20)  # rounds the bottom left corner

The image of the documentation shows it too: its code rounds the BOTTOM_RIGHT corner of the green rectangle and the TOP_LEFT and BOTTOM_LEFT corners of the yellow square, but the image shows the bottom left corner and the right corners rounded. In _draw_rounded_rect, each corner name moved the points and drew the arc of the corner on the other side; the names in the conditions are now swapped back (and the comments of the arcs fixed to match).

• test_round_corners_rect_corner_position checks, for each corner name, that the arc is drawn in that corner of the rectangle; it fails for all four names before the fix.

• The three class_round_corners_rect*.pdf reference files are regenerated: in their second row, each rectangle now has the named corner rounded. round_corners=True is unchanged.

• The documentation example passed round_corners=("BOTTOM_RIGHT"), a string rather than a tuple, which raises ValueError: B is not a valid Corner; it now passes ("BOTTOM_RIGHT",), and round_corners_rectangles.png is regenerated from it.

• A unit test is covering the code added / modified by this PR

• A mention of the change is present in CHANGELOG.md

• This PR is ready to be merged