src/api-impl: use skia instead of cairo

Using the C API provided by SkiaSharp's skia fork instead of using cairo
significantly improves performance. The API is also closer to the android
Canvas API, which makes the implementation more straightforward.

src/api-impl-jni/generated_headers/android_graphics_Paint.h

@@ -0,0 +1,37 @@
+/* DO NOT EDIT THIS FILE - it is machine generated */
+#include <jni.h>
+/* Header for class android_graphics_Paint */
+
+#ifndef _Included_android_graphics_Paint
+#define _Included_android_graphics_Paint
+#ifdef __cplusplus
+extern "C" {
+#endif
+/*
+ * Class:     android_graphics_Paint
+ * Method:    native_constructor
+ * Signature: ()J
+ */
+JNIEXPORT jlong JNICALL Java_android_graphics_Paint_native_1constructor
+  (JNIEnv *, jobject);
+
+/*
+ * Class:     android_graphics_Paint
+ * Method:    native_set_color
+ * Signature: (JI)V
+ */
+JNIEXPORT void JNICALL Java_android_graphics_Paint_native_1set_1color
+  (JNIEnv *, jobject, jlong, jint);
+
+/*
+ * Class:     android_graphics_Paint
+ * Method:    native_get_color
+ * Signature: (J)I
+ */
+JNIEXPORT jint JNICALL Java_android_graphics_Paint_native_1get_1color
+  (JNIEnv *, jobject, jlong);
+
+#ifdef __cplusplus
+}
+#endif
+#endif