pen: Rework public API.

This changes the API in various ways, and updates the backends for this.

Overall, this is a massive simplification of the API, as most future backends
can't support the previously-offered API.

This also removes the testautomation pen code (not only did these interfaces
change completely, it also did something no other test did: mock the internal
API), and replaces testpen.c with a different implementation (the existing
code was fine, it was just easier to start from scratch than update it).

test/testautomation.c

@@ -34,9 +34,6 @@ static SDLTest_TestSuiteReference *testSuites[] = {
     &mainTestSuite,
     &mathTestSuite,
     &mouseTestSuite,
-#if !defined(SDL_PLATFORM_IOS) && !defined(SDL_PLATFORM_TVOS)
-    &penTestSuite,
-#endif
     &pixelsTestSuite,
     &platformTestSuite,
     &propertiesTestSuite,