this fixes another NaN caused in cases where the `end` vector of the
rotate mode is at y = 0. this might be caused by various factors,
notably again when the cursor starts at 0;0
thanks again @RGBCube
when the plugin is loaded whilst the cursor pos is at 0;0, the `end`
vector is being normalized allthough it will contain 0;0. this will
produce a division by zero, causing the rotation to be set to NaN,
ultimately causing the cursor not to render because of a NaN'd
projection matrix
thanks for reporting and testing @RGBCube
