fix: -Wsign-compare warnings (#336)

mixing signed and unsigned types in comparison can end up having
unintended results. for example:

	if (-1 < 1U)
		printf("true\n");
	else
		printf("false\n");

previously we silenced these warnings, instead just fix them properly
via necessary casting, and in cases where the value cannot be negative
(e.g width/height members) make them unsigned.

Reviewed-on: https://codeberg.org/nsxiv/nsxiv/pulls/336
Reviewed-by: explosion-mental <explosion-mental@noreply.codeberg.org>

.woodpecker/CFLAGS

@@ -10,4 +10,4 @@
 -Wbad-function-cast -Wdeclaration-after-statement
 -Wmissing-prototypes -Wstrict-prototypes
 # silence
--Wno-sign-compare -Wno-unused-parameter -Wno-missing-field-initializers
+-Wno-unused-parameter -Wno-missing-field-initializers