Simple shadows for FlatTheme

[dhardy]

Implements basic shadows. CC #238, @hummingly.

Contrary to the discussion in #238, these are implemented using simple shaders, without even a mask (just relying on the draw order). Implications:

• Using custom size, offset and colours (glow effects) is easy
• Lower GPU load & memory usage, though this is probably irrelevant
• Shadows may look incorrect with too small margins (e.g. if a button is directly inside a frame, the frame will be drawn over the button's shadow)
• Sub-menus appear over parents instead of alongside with merged shadows
• To get outer curvature correct, shadows start inside the shape; with circles they start further inside than frames, hence the strongest visible part of the shadow is fainter on circles
• Outer curvature is currently squashed/stretched instead of simply offset; I could change this but not sure which is better.

I may leave shadows like this for now, and later implement post-processing only for pop-up menus — though those pop-ups are supposed to be rendered by the system compositor anyway (winit doesn't support this yet).

Forgot to show it in the video, but the under-bar on the edit box now has a small shadow too, but only on hover and focus.

[dhardy]

I reduced the shadows a bit over the above: