SDL2: updated to 2.0.8

2.0.8:

General:
* Added SDL_fmod() and SDL_log10()
* Each of the SDL math functions now has the corresponding float version
* Added SDL_SetYUVConversionMode() and SDL_GetYUVConversionMode() to control the formula used when converting to and from YUV colorspace. The options are JPEG, BT.601, and BT.709

Windows:
* Implemented WASAPI support on Windows UWP and removed the deprecated XAudio2 implementation
* Added resampling support on WASAPI on Windows 7 and above

Windows UWP:
* Added SDL_WinRTGetDeviceFamily() to find out what type of device your application is running on

Mac OS X:
* Added support for the Vulkan SDK for Mac:
  https://www.lunarg.com/lunarg-releases-vulkan-sdk-1-0-69-0-for-mac/
* Added support for OpenGL ES using ANGLE when it's available

Mac OS X / iOS / tvOS:
* Added a Metal 2D render implementation
* Added SDL_RenderGetMetalLayer() and SDL_RenderGetMetalCommandEncoder() to insert your own drawing into SDL rendering when using the Metal implementation

iOS:
* Added the hint SDL_HINT_IOS_HIDE_HOME_INDICATOR to control whether the home indicator bar on iPhone X should be hidden. This defaults to dimming the indicator for fullscreen applications and showing the indicator for windowed applications.

iOS / Android:
* Added the hint SDL_HINT_RETURN_KEY_HIDES_IME to control whether the return key on the software keyboard should hide the keyboard or send a key event (the default)

Android:
* SDL now supports building with Android Studio and Gradle by default, and the old Ant project is available in android-project-ant
* SDL now requires the API 19 SDK to build, but can still target devices down to API 14 (Android 4.0.1)
* Added SDL_IsAndroidTV() to tell whether the application is running on Android TV

Android / tvOS:
* Added the hint SDL_HINT_TV_REMOTE_AS_JOYSTICK to control whether TV remotes should be listed as joystick devices (the default) or send keyboard events.

Linux:
* Added the hint SDL_HINT_VIDEO_X11_NET_WM_BYPASS_COMPOSITOR to control whether the X server should skip the compositor for the SDL application. This defaults to "1"
* Added the hint SDL_HINT_VIDEO_DOUBLE_BUFFER to control whether the Raspberry Pi and KMSDRM video drivers should use double or triple buffering (the default)

(adam)