Flat markers should have screen-centered vanishing point #5218

As seen in #3276 (comment), the vanishing point for a flat marker view on a tilted map is always somewhere directly above the marker view. So in the screenshot above, the flag to the right is symmetrical across the y axis, whereas you’d expect both the left and right sides of the flag to parallel the north-south streets they’re being drawn atop:

If the perspective transformation is executed correctly, you shouldn’t be able to tell that the flag is a marker view. As you pan the map from side to side, the streets shouldn’t appear to be moving relative to the flag. In graphics terms, the vanishing point for the transform should be horizontally centered; it shouldn’t vary by the x position of the marker view.

Due to #5090, we haven’t run into this issue on iOS yet, but it’s something to be aware of.

/cc @tobrun @bleege @boundsj @friedbunny