Zero-copying on Android

There's a few things that make zero-copying difficult on Android:

• Buffers are in RGBX format instead of BGRX. Should be fixed by github.com/rust-windowing/softbuffer/pull/289.
• Buffers have stride. Should be fixed by https://github.com/rust-windowing/softbuffer/pull/315.
• Buffers are exposed as &mut [MaybeUninit<u8>], see NativeWindowBufferLockGuard::bytes. Is this really necessary, or could we rely on the buffer being zero-initialized? If not, we'll have to do a zeroing step ourselves in buffer_mut.
• The surface is both unlocked and presented in NativeWindowBufferLockGuard::drop, whereas Softbuffer allows these two steps to be separate (e.g. you can Drop an in-progress Buffer<'_> and request a new one in Surface::buffer_mut, with no side-effects).

Maybe the SurfaceControl stuff @MarijnS95 is working on will allow fixing the last point?

Start with NativeWindowBufferLockGuard::bytes and drop, then trace Softbuffer's Surface::buffer_mut behavior for comparison. Determine whether Android buffers can avoid an explicit zeroing step and how unlock and present should be separated; done means both remaining unchecked Android zero-copy concerns are resolved consistently.