Texture modifiers and imageCleanTransparent are too slow

[Desour]

Performance issue.

This issue appears in at least two places:

• Joining a server takes longer. (Related: Client hangs on startup if too many texture modifiers are used in a game or mod #11829)
• Loading objects with new textures (e.g. signs) freezes the client for a short time.

How to test

• Enable any texture filter stuff, like mip mapping, to trigger [applyfiltersformesh/imageCleanTransparent stuff.
• Install signs_lib (https://github.com/mt-mods/signs_lib) and some luacmd fork (https://github.com/Desour/luacmd) in a mtg world.
• Create some signs with different text on each: /lua local heren = here:round() for x = 0, 8 do for y = 0, 8 do for z = 0, 8 do local p = heren:offset(x,y,z) core.set_node(p, {name="default:sign_wall_wood",param2=2}) core.get_meta(p):set_string("text", p:to_string()..string.rep("\nAAAAAAA",5)) end end end
• Rejoin.
• Notice how the client freezes. when the sign objects load.
  Edit: To measure, show the F5 profiler graphs, and look at "Time non-rendering [us]".

Here's a flamegraph: