Includes:
* Fix GPUSurfaceGL includes on Linux (flutter/engine#4849)
* Slightly improve the docs for the internal _futurize method (flutter/engine#4847)
* Fix supported color type check on iOS simulators (flutter/engine#4846)
* Fix GPUSurfaceGL includes on macOS (flutter/engine#4844)
* Handle Apple-specific GLES headers (flutter/engine#4843)
* [fuchsia] Fix up includes of Zircon headers (flutter/engine#4842)
* VulkanSurface and GPUSurfaceGL no longer use GrPixelConfig (flutter/engine#4814)