Add `SkiaSurface`, as a base for all the Skia-backed core provider implementations

The decision is almost made, Skia will be aggressively used to fix a lot of long-standing Kivy issues, and Skia-backed core provider implementations will be a huge part of Kivy 3.0.0

Avoiding DRY violations since day 0, and advertising reuse of the code is a must.

SkiaSurface can likely live (but feel free to propose a better place) in kivy/lib/skia, and serve as the surface (via Texture or FBO) for Skia-backed core provider implementations.

Some examples:

• SkiaTextLayout for https://github.com/kivy/kivy/issues/8574
• SkiaSVG for https://github.com/kivy/kivy/issues/8573
• SkiaLottie