Viewport Compositor: Port Time node

This patch ports the Time node to the viewport compositor. The current
frame number was added to the View UBO in order to be accessed by the
shader.

Viewport Compositor: Port Luminance Key node

This patch ports the Luminance Key node to the viewport compositor. The
shader is a straightforward port of the compositor code.

Viewport Compositor: Port Distance Key node

This patch ports the Distance Key node to the viewport compositor. The
shader is a straightforward port of the compositor code.

Viewport Compositor: Port Difference Key node

This patch ports the Difference Key node to the viewport compositor. The
shader is a straightforward port of the compositor code.

Viewport Compositor: Port Color Key node

This patch ports the Color Key node to the viewport compositor. The
shader is a straightforward port of the compositor code.

Viewport Compositor: Port Color Spill node

This patch ports the Color Spill node to the viewport compositor. The
shader is a straightforward port of the compositor code.

Viewport Compositor: Port Chroma Key node

This patch ports the Chroma Key node to the viewport compositor. The
shader is a straightforward port of the compositor code.

Viewport Compositor: Port Value node

This patch ports the Value node to the viewport compositor.

Viewport Compositor: Port RGB node

This patch ports the RGB node to the viewport compositor.

Viewport Compositor: Port Box Mask node

This patch ports the Box Mask node to the viewport compositor.

Viewport Compositor: Port Channel Key node

This patch ports the Channel Key node to the viewport compositor. The
shader is a straightforward port of the compositor code.

Viewport Compositor: Port Ellipse Mask node

This patch ports the Ellipse Mask node to the viewport compositor.

Viewport Compositor: Port Split Viewer node

This patch ports the Split Viewer node to the viewport compositor.

Viewport Compositor: Port Vector Curves node

This patch ports the Vector Curves node to the viewport compositor. A
variant of the existing vector_curves was added to work without mixing
and the inputs were reduced to vec3 because vec4 are superfluous.

Viewport Compositor: Port Set Alpha node

This patch ports the Set Alpha node to the viewport compositor. The
shader is a straightforward port of the compositor code.

Viewport Compositor: Port Map Value node

This patch ports the Map Value node to the viewport compositor. The
shader is a straightforward port of the compositor code.

Viewport Compositor: Port Convert Alpha node

This patch ports the Convert Alpha node to the viewport compositor. The
shader is a straightforward port of the compositor code.

Viewport Compositor: Port Normal node

This patch ports the Normal node to the viewport compositor. The shader
is a straightforward port of the compositor code.

Viewport Compositor: Port Separate/Combine nodes

This patch ports the Separate/Combine nodes to the viewport compositor.
The shader is a straightforward port of the compositor code.

Viewport Compositor: Port Color Ramp node

This patch ports the Color Ramp node to the viewport compositor. The
shading color ramp shader was moved into a common directory to be used
by both materials and the compositor.