Based on API review, PlatformBuilder becomes BackendSelector with
a slightly smaller API surface but more options, such as selecting
Metal or Vulkan rendering.
Commit 3cb636169506262d96ec2bc53a617714d5ab068d in femtovg switched to
using WeblGL to (needed for tiling). This means the GraphicsAPI::WebGL
context type needs to be updated, too. Also, the canvas_id was always
empty, but needs to be initialized from the id of the element we're
using.
Use a CStr instead of str, to avoid an extra CString allocation. The underlying operating system API for
this expects a zero terminated C String.
Fixes#1943
