[BUG FIX] Mouse drag crashing the viewer in edge cases (#644)

* fix: crash in viewer when dragging camera before initial click

Previously, viewer would crash if the `on_mouse_drag` callback was
triggered before `on_mouse_click`. This happened because the initial
mouse press would set `_pdown` attribute, which was initialized to
`None` when the `Trackball` instance is created, resulting in a
`TypeError` during camera drag calculations.

This fix ensures the `_pdown` value is properly initialized to
prevent the crash in some edge cases like window snap resizing.
It also removes the attribute access to `_pdown` with getattr
since it is always initialized.

* chore: fix typo in comment

Author: eratc

genesis/ext/pyrender/trackball.py

@@ -46,7 +46,7 @@ def __init__(self, pose, size, scale, target=np.array([0.0, 0.0, 0.0])):
 
         self._state = Trackball.STATE_ROTATE
 
-        self._pdown = None  # Initialize _pdown attribute to None
+        self._pdown = np.array([0.0, 0.0], dtype=np.float32)
 
     @property
     def pose(self):
@@ -88,7 +88,7 @@ def down(self, point):
         self._target = self._n_target
 
     def drag(self, point):
-        """Update the tracball during a drag.
+        """Update the trackball during a drag.
 
         Parameters
         ----------
@@ -98,9 +98,7 @@ def drag(self, point):
             motion between this point and the one marked by down().
         """
         point = np.array(point, dtype=np.float32)
-        # get the "down" point defaulting to current point making
-        # this a no-op if the "down" event didn't trigger for some reason
-        dx, dy = point - getattr(self, "_pdown", point)
+        dx, dy = point - self._pdown
         mindim = 0.3 * np.min(self._size)
 
         target = self._target