gsk/vulkan/renderpass: Factor out node implementations

gsk/vulkan/renderpass: Factor out node implementations

This is mostly a cosmetic change, and the goal is twofold:

 1. Make it easier to spot unimplemented render node types; and
 2. Prepare for a small rework

The implementation for each node now lives in specific functions,
like the GL renderer; unlike the GL renderer, however, we use a
node type vtable to map GskRenderNodeType → implementation. Render
node without an implementation map to NULL, and use the fallback
implementation. Render nodes that fail any check and return FALSE
also use fallback implementation.