UI: implement 'pumblend' option for semi-transparent popupmenu

Why?

- Because we can.
- Because the TUI is just another GUI™
- Because it looks kinda nice, and provides useful context like 1 out of 100
  times

Complies with "don't pay for what you don't use".

Some crashes for resizing were unfolded, add tests for those.

src/nvim/popupmnu.c

@@ -44,6 +44,7 @@ static int pum_col;                 // left column of pum
 static bool pum_is_visible = false;
 static bool pum_is_drawn = false;
 static bool pum_external = false;
+static bool pum_invalid = false;  // the screen was just cleared
 
 static ScreenGrid pum_grid = SCREEN_GRID_INIT;
 
@@ -360,12 +361,14 @@ void pum_redraw(void)
   grid_assign_handle(&pum_grid);
   bool moved = ui_comp_put_grid(&pum_grid, pum_row, pum_col-col_off,
                                 pum_height, grid_width);
+  bool invalid_grid = moved || pum_invalid;
+  pum_invalid = false;
 
   if (!pum_grid.chars
       || pum_grid.Rows != pum_height || pum_grid.Columns != grid_width) {
-    grid_alloc(&pum_grid, pum_height, grid_width, !moved, false);
+    grid_alloc(&pum_grid, pum_height, grid_width, !invalid_grid, false);
     ui_call_grid_resize(pum_grid.handle, pum_grid.Columns, pum_grid.Rows);
-  } else if (moved) {
+  } else if (invalid_grid) {
     grid_invalidate(&pum_grid);
   }
 
@@ -806,6 +809,17 @@ bool pum_drawn(void)
   return pum_visible() && !pum_external;
 }
 
+/// Screen was cleared, need to redraw next time
+void pum_invalidate(void)
+{
+  pum_invalid = true;
+}
+
+void pum_recompose(void)
+{
+  ui_comp_compose_grid(&pum_grid);
+}
+
 /// Gets the height of the menu.
 ///
 /// @return the height of the popup menu, the number of entries visible.