Add stricter check for corner markers

This MR adds a few new checks and improvements for the corner marker detection:

• Searches for corner marker corner by corner. This makes check_corner_keypoints() obsolete.
• Ensures the two lines of the marker are perpendicular.
• Ensures the dimensions of the blob of the marker are correct.
• Ensures the angle of the corner marker is within 15 degrees of the original.
• Slightly increases the threshold for converting an image to binary to counter aliasing (only for corner markers).

Hopefully closes #429 (closed)