guiScalingFilter: prevent infinite loop caused by negative background9[] size

[SmallJoker]

https://gitlab.com/Df458/cartographer + https://gitlab.com/Df458/cartographer-mtg create a formspec where a background9[] element has a negative Y size. This results in sh=-1.000 and invalid texture dimensions in imageScaleNNAA which then locks up Minetest.

0x0000555555713275 in imageScaleNNAA (src=src@entry=0x55555687d140, srcrect=..., dest=dest@entry=0x555556879ec0) at minetest/src/client/imagefilters.cpp:191
191			if (minsx > maxsx)
(gdb) bt
#0  0x0000555555713275 in imageScaleNNAA (src=src@entry=0x55555687d140, srcrect=..., 
    dest=dest@entry=0x555556879ec0)
    at minetest/src/client/imagefilters.cpp:191
#1  0x0000555555706af8 in guiScalingResizeCached (driver=0x555555ed1b40, src=<optimized out>, 
    srcrect=..., destrect=...) at minetest/src/client/guiscalingfilter.cpp:121

This PR now skips invalid sized textures and outputs an error message in case it was not checked previously.

To do

This PR is Ready for Review.

How to test

1. Enable the GUI scaling filter

minetest.register_chatcommand("fs", {
	func = function(name, param)
		minetest.show_formspec(name, "freeze",
			"size[4,4]" ..
			"background9[2,2;-1,-1;default_cobble.png;false;20]" .. -- problematic
			"background[0,0;-4,-4;default_cobble.png]" -- is OK
		)
	end
})

3. compare master vs PR

[sfan5]

lgtm