Transform::flyTo invalid zoom checks

[brunoabinader]

Whilst Transform::easeTo interpolates the zoom from initial to end values (thus never breaching the end value threshold), Transform::flyTo has a "bounce" effect on zoom in some circumstances, by applying a formula on each frame rendering that calculates a new zoom value.

In a particular case e.g. executing flyTo with camera options' zoom level set to the maximum value, the zoom value sometimes goes above the maximum zoom threshold, causing the transform state to become invalid, thus causing this crash in tileCover:

mbgl-glfw: ../../../src/mbgl/util/tile_cover.cpp:159: std::vectormbgl::UnwrappedTileID
mbgl::util::tileCover(const mbgl::TransformState&, int32_t): Assertion `state.valid()' failed.

This PR introduces a clamping to scale boundaries in TransformState::setLatLngZoom, and also a check if the frameZoom is NaN in flyTo (which can occur if the transform state size is empty).

[ivovandongen]

👍 Nice timing, ran into this just yesterday when testing some unrelated changes

[1ec5]

Awesome! For reference, I was running into this issue as well in the iPhone 7 simulator (portrait orientation):

1. Go to z4 around Central Europe.
2. Set the simulated location to Apple, so that the user’s location is in Cupertino.
3. Click the location arrow in the toolbar to fly to the user’s location.