Fix rotate_around_point to rotate about the source point

[uttam12331]

Summary

arcade.math.rotate_around_point(source, target, angle) rotates target around source, but it adds the rotated offset back to target instead of to the center of rotation source. As a result the returned point is placed incorrectly and its distance from the center is not preserved.

Reproduction

from arcade.math import rotate_around_point

# Rotate (1, 0) around the origin by 90 degrees.
print(rotate_around_point((0.0, 0.0), (1.0, 0.0), 90.0))
# actual:   (1.0, 1.0)   -> distance sqrt(2) from the center
# expected: (0.0, 1.0)   -> distance 1.0 from the center

Any rotation where the source and target differ lands on the wrong point.

Fix

The offset (dx, dy) is the rotated source -> target vector, so it must be added to the center of rotation source, not to target:

-    return target[0] + dx, target[1] + dy
+    return source[0] + dx, source[1] + dy

Tests

Added two regression tests in tests/unit/test_math.py that are independent of the clockwise/counter-clockwise convention:

• the rotated point keeps its distance from source for several angles, and
• a 180 degree rotation reflects target through source.

Both fail on the previous behavior and pass with the fix.