perf(extmarks): add metadata for efficient filtering of special decorations

This expands on the global "don't pay for what you don't use" rules for
these special extmark decorations:

- inline virtual text, which needs to be processed in plines.c when we
  calculate the size of text on screen
- virtual lines, which are needed when calculating "filler" lines
- signs, with text and/or highlights, both of which needs to be
  processed for the entire line already at the beginning of a line.

This adds a count to each node of the marktree, for how many special
marks of each kind can be found in the subtree for this node. This makes
it possible to quickly skip over these extra checks, when working in
regions of the buffer not containing these kind of marks, instead of
before where this could just be skipped if the entire _buffer_
didn't contain such marks.

src/nvim/buffer_defs.h

@@ -701,10 +701,6 @@ struct file_buffer {
 
   MarkTree b_marktree[1];
   Map(uint32_t, uint32_t) b_extmark_ns[1];         // extmark namespaces
-  size_t b_virt_text_inline;                       // number of inline virtual texts
-  size_t b_virt_line_blocks;    // number of virt_line blocks
-  size_t b_signs;               // number of sign extmarks
-  size_t b_signs_with_text;     // number of sign extmarks with text
 
   // array of channel_id:s which have asked to receive updates for this
   // buffer.