Client: upscale [mask or base image (#15205)

This improves texture pack compatibility. Masks are expected to be of the same
size as the base texture. This change upscales the smaller texture if needed.
The behaviour is now the same as a.png^b.png and a.png^[overlay:b.png (to mention a few).

games/devtest/mods/testnodes/textures.lua

@@ -52,6 +52,12 @@ minetest.register_node("testnodes:fill_positioning_reference", {
 	groups = {dig_immediate = 3},
 })
 
+minetest.register_node("testnodes:modifier_mask", {
+	description = S("[mask Modifier Test Node"),
+	tiles = {"testnodes_128x128_rgb.png^[mask:testnodes_mask_WRGBKW.png"},
+	groups = {dig_immediate = 3},
+})
+
 -- Node texture transparency test
 
 local alphas = { 64, 128, 191 }