Celerity::Render2d: Implement viewports with optional viewport-to-viewport blending.

[KonstantinTomashevich]

Currently Celerity::Render2d works with predefined full-window viewport. We need to change that by adding viewports feature:

• Any number of Viewport instances can be added.
• Rendering is driven by Viewport's.
• Viewports are sorted by sort index.
• Viewport coordinates are configured in pixels.
• Viewports can overlap each other and can be blended: blending is useful for rendering something like UI in separate viewport.

[KonstantinTomashevich]

With how views in BGFX work, it seems that it is impossible to implement alpha blending between views: only overlapping is possible. It should be fine for UI right now, but we should also plan to support transparent UI in the future.