Formspec: allow lists to change size and existence while the formspec is open

[Desour]

• Goal of the PR: Fix a regression.
• How does the PR work? (All functional are changes listed here to make reviewing easier.)
  • GUIFormSpecMenu::parseList no longer checks for the existence of the inventory.
  • GUIInventoryList::m_geom is now exactly the geom given in the formspec string. If only a smaller inventory should be drawn, that is decided in GUIFormSpecMenu::draw. (Inventories can change their size any time.) Hence there's no more trimming of geom.
  • To avoid a flood of warnings, there's now GUIInventoryList::m_already_warned. Only one warning is written in a row.
  • GUIInventoryList::getItemIndexAtPos is improved. It now also returns -1 if there can not be an item at the specified position because the inventory list is too small.
• Fixes Inventory not being rendered #9640.

To do

This PR is a Ready for Review.

How to test

• Use the code from here: Inventory not being rendered #9640 (comment)
• Try the inventorybags mod.

[SmallJoker]

Took the code from #9640 (comment)
After adding minetest.after(0, ...) the error still did not show up: (formspec packet was faster)

finished.
Client: Detached inventory update: "fs_test_detached_inv", mode=update
Client: Detached inventory update: "fs_test_detached_inv", mode=update
Game::handleClientEvent_ShowFormSpec

A delay of >1 server step finally let the warning appear:

finished.
Game::handleClientEvent_ShowFormSpec
2020-04-17 19:23:36: WARNING[Main]: GUIFormSpecMenu::parseList(): The inventory location "detached:fs_test_detached_inv" doesn't exist                                                                                                              
Client: Detached inventory update: "fs_test_detached_inv", mode=update
Client: Detached inventory update: "fs_test_detached_inv", mode=update

PR works as expected. The inventory list shows up after the formspec is already rendered.