fix(input): mouse_dx/dy report no motion on the first frame or across a cursor-mode change

The delta was this frame's position minus the last, and the last started at 0,0, so the
first frame reported the cursor's whole distance from the corner as motion. A cursor-mode
change did the same, switching between a locked cursor's virtual reticle and the real
cursor. Maroon Lake's camera adds mouse_dy to its pitch and came up pointing at the ground.

examples/library/input_mouse_rebase.ludic covers both cases, plus ordinary motion and
re-setting the mode already in force.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

examples/library/input_mouse_rebase.ludic

@@ -0,0 +1,44 @@
+# input_mouse_rebase.ludic — Input.mouse_dx/dy report no motion on the first frame and on the
+# frame a cursor-mode change lands, and ordinary motion on every other frame.
+#
+# The delta is this frame's position minus the last. Before any position has been read the "last"
+# one is 0,0, and across a mode change it belongs to another source (a locked cursor's virtual
+# reticle against the real cursor) - both used to come out as a jump the size of the screen.
+# Driven headless by injection:
+#   bin/ludic examples/library/input_mouse_rebase.ludic  ->  0 0 5 -2 0 0 4 3 6 0 0 0
+program InputMouseRebase {
+  entry {
+    # the first frame: the cursor is already at 300,400, which is not 300,400 of motion
+    Input.set_mouse(300, 400, 0, 0)
+    Input.poll()
+    print(Input.mouse_dx())              # 0
+    print(Input.mouse_dy())              # 0
+    # ordinary motion
+    Input.set_mouse(305, 398, 0, 0)
+    Input.poll()
+    print(Input.mouse_dx())              # 5
+    print(Input.mouse_dy())              # -2
+    # a mode change: the next position is from another source
+    Input.cursor_mode(2)
+    Input.set_mouse(900, 100, 0, 0)
+    Input.poll()
+    print(Input.mouse_dx())              # 0
+    print(Input.mouse_dy())              # 0
+    Input.set_mouse(904, 103, 0, 0)
+    Input.poll()
+    print(Input.mouse_dx())              # 4
+    print(Input.mouse_dy())              # 3
+    # asking for the mode already in force is not a change
+    Input.cursor_mode(2)
+    Input.set_mouse(910, 103, 0, 0)
+    Input.poll()
+    print(Input.mouse_dx())              # 6
+    print(Input.mouse_dy())              # 0
+    # and back: no jump from the reticle to the real cursor
+    Input.cursor_mode(0)
+    Input.set_mouse(10, 10, 0, 0)
+    Input.poll()
+    print(Input.mouse_dx())              # 0
+    print(Input.mouse_dy())              # 0
+  }
+}