Updates to Map::{renderSync,nudgeTransitions}

[brunoabinader]

Given the example in #1975 where a custom animation (not triggered by an input event) triggers a Map repaint, Map should be self-sufficient in terms of data to avoid triggering View invalidations in a burst.

Important changes:

• Map::renderSync() no longer provides a return parameter.
• Map::nudgeTransitions() no longer requires a forceRerender parameter.
• MapContext::renderSync() should be ideally returning void, however for performance reasons we no w only return the value of MapContext::isLoaded() to avoid a second invokeSync() from Map to MapContext.
• Removed Transform::needsTransition() - Transform::updateTransitions() already checks for the transition callback.
• Moved StillImageCallback to its own header to avoid duplicated definitions in Map and MapContext headers.
• Added Update::Repaint enum to notify MapContext that all we require is a style update, followed by a view invalidation.
• Coalesce view invalidations in MapContext::invalidateView() to avoid flooding the client with redundant invalidation calls.
• Updated public API changes in the ports code accordingly.

/cc @incanus @kkaefer @jfirebaugh @tmpsantos @tmcw

Fixes #1975.