Separate src/driver into its own library

In this first step, we just "blindly" move everything under src/driver
to a new library libbackend.a. And all headers are moved under
private/backend.

Note that "driver" is renamed "backend", but namespaces are unchanged for now.

Later we'll have to untangle the actual private headers from public
ones and ideally not have any private headers.

android/filament-android/src/main/cpp/Texture.cpp

@@ -23,7 +23,7 @@
 #include <android/bitmap.h>
 #endif
 
-#include <filament/driver/BufferDescriptor.h>
+#include <backend/BufferDescriptor.h>
 #include <filament/Engine.h>
 #include <filament/Stream.h>
 #include <filament/Texture.h>