Add KTX and ETC1 texture support by @Triang3l

https://github.com/xonotic/netradiant/pull/1
https://gitlab.com/xonotic/netradiant/-/issues/5

This pull request adds support for Khronos Textures to NetRadiant and Q3Map2, with OpenGL ES 2.0 formats, Ericsson Texture Compression version 1, and BGR/BGRA.

The patent-free Ericsson Texture Compression format will be used in the next version of Warsow to significantly reduce VRAM usage on mobile GPUs and the integrated GPU of Intel Broadwell.

Makefile

@@ -583,6 +583,7 @@ $(INSTALLDIR)/q3map2.$(EXE): \
 	tools/quake3/q3map2/vis.o \
 	tools/quake3/q3map2/writebsp.o \
 	libddslib.$(A) \
+	libetclib.$(A) \
 	libfilematch.$(A) \
 	libl_net.$(A) \
 	$(if $(findstring Win32,$(OS)),icons/q3map2.o,) \
@@ -814,6 +815,10 @@ libddslib.$(A): CPPFLAGS_EXTRA := -Ilibs
 libddslib.$(A): \
 	libs/ddslib/ddslib.o \
 
+libetclib.$(A): CPPFLAGS_EXTRA := -Ilibs
+libetclib.$(A): \
+	libs/etclib.o \
+
 $(INSTALLDIR)/q3data.$(EXE): LIBS_EXTRA := $(LIBS_XML) $(LIBS_GLIB) $(LIBS_ZLIB)
 $(INSTALLDIR)/q3data.$(EXE): CPPFLAGS_EXTRA := $(CPPFLAGS_XML) $(CPPFLAGS_GLIB) $(CPPFLAGS_ZLIB) -Itools/quake3/common -Ilibs -Iinclude
 $(INSTALLDIR)/q3data.$(EXE): \
@@ -839,6 +844,7 @@ $(INSTALLDIR)/q3data.$(EXE): \
 	tools/quake3/q3data/stripper.o \
 	tools/quake3/q3data/video.o \
 	libfilematch.$(A) \
+	libetclib.$(A) \
 	libl_net.$(A) \
 	libmathlib.$(A) \
 	$(if $(findstring Win32,$(OS)),icons/q3data.o,) \
@@ -1039,9 +1045,11 @@ $(INSTALLDIR)/modules/image.$(DLL): \
 	plugins/image/dds.o \
 	plugins/image/image.o \
 	plugins/image/jpeg.o \
+	plugins/image/ktx.o \
 	plugins/image/pcx.o \
 	plugins/image/tga.o \
 	libddslib.$(A) \
+	libetclib.$(A) \
 
 $(INSTALLDIR)/modules/imageq2.$(DLL): CPPFLAGS_EXTRA := -Ilibs -Iinclude
 $(INSTALLDIR)/modules/imageq2.$(DLL): \