API BREAKAGE: this change aims to fix multi-view support

[pixelflinger]

What has changed:

• View doesn't have a notion of clear color anymore
• View doesn't have a notion of discard flags anymore
• The clear color and color-buffer discard/clear flags are moved to Renderer
• Skybox can now be set to a constant color
• View have a blend-mode

What does is all mean:
"Clearing" (i.e.) setting its background is now handled by Skybox, by
setting a constant color to the Skybox. This should take care of
drawing views side by side.

When a view needs to be drawn on top of another, it's BlendMode needs to
be set to TRANSLUCENT and of course and, generally, it wither won't have
a skybox, or will have one that sets some translucent pixels.

As an optimization, a View with BlendMode::OPAQUE will have its
background cleared with the color specified in Renderer.

If the SwapChain already has some content, it's now possible to set
the Renderer to not discard the content, together with TRANSLUCENT views,
it's possible to draw on top of that content.

It is NOT possible to share depth/stencil buffers between views.

Fixes: #2369, #2372, #2364

[bejado]

A couple iOS samples use the old clear API and are failing to compile:

ios/samples/transparent-rendering/transparent-rendering/FilamentView.mm
167:    filaView->setClearColor({0.0f, 0.0f, 0.0f, 0.0f});

ios/samples/hello-triangle/hello-triangle/FilamentView.mm
156:    filaView->setClearColor({0.1, 0.125, 0.25, 1.0});