[greensopinion]

MapOptions.maxPitch (default 0) allows pitch up to 60°. Ground layers
draw in perspective; markers and labels face the viewer. Adds
MapController.pitch and BillboardLayerTransformer.

I wrote this with Claude Code (Claude Opus 5.5). I directed the design,
reviewed the code, and tested it on a Mac, an iPhone 13, an iPhone 15 Pro
Max, a Pixel 4a (5G), and in Chrome.

This is the first of 5 PRs that add pitch. The others are:

• Pitch gestures: two-finger vertical drag on touch screens, and Ctrl-drag
  on desktop.
• Camera fits that account for pitch, and contain constraints that ignore
  pitch again, as in MapLibre GL JS.
• A fix, independent of pitch: a tile whose URL matched an aborted load
  could show the aborted load's transparent image.
• Tile level of detail on pitched maps: far tiles load at a coarser zoom,
  and only the tiles that cover the visible ground are loaded.
• A sixth PR, which limits pitch near the horizon, will follow

[greensopinion]

Replaced by #2283, which has the same commit from a branch in this
repository, so that it can be the bottom of a stack with #2282. GitHub's
stacks can't include a PR from a fork.