app_server: add semi-transparent window support

[X512]

Today many 3rd-party applications use semi-transparent windows for decorations, shadows or visual effects that currently looks bad in Haiku. For sone software it may be unrealistic and/or undesired to turn off using semi/transparent parts. Many people prefer themes with semi-transparent parts that are currently impossible to implement.

Proposed implementation strategy:

1. Introduce Surface objects for HWInterface that can store raster buffer and be composited with alpha channel blending. Surfaces have Z-order relations and displayed on top of screen framebuffer. In future hardware compositing acceleration can be considered.

2. Allocate and attach Surface to a window that has semi-transparent parts and redirect rendering to that surface instead of screen framebuffer.

3. Modify Desktop visible region calculation so window with attached Surface will be not subtracted from windows with lower Z-order. Windows with upper Z-order and no attached Surface will be subtracted from window visible region. Window visible region will be also set to Surface object.