etcpak: We only need the compression code, remove rest of etcpak app

We do our own image loading, threading, and memory management in Godot already,
so the only components we need from etcpak (at least as of now) are the
`Compress*` methods defined in `ProcessDxtc.cpp` and `ProcessRGB.cpp`.

So we don't need to compile or vendor the rest.

modules/etcpak/SCsub

@@ -11,31 +11,15 @@ thirdparty_obj = []
 
 thirdparty_dir = "#thirdparty/etcpak/"
 thirdparty_sources = [
-    "Bitmap.cpp",
-    "BitmapDownsampled.cpp",
-    "BlockData.cpp",
-    "ColorSpace.cpp",
-    "DataProvider.cpp",
-    "Debug.cpp",
     "Dither.cpp",
-    "Error.cpp",
-    "mmap.cpp",
     "ProcessDxtc.cpp",
     "ProcessRGB.cpp",
-    "System.cpp",
     "Tables.cpp",
-    "TaskDispatch.cpp",
-    "Timing.cpp",
-    "lz4/lz4.c",
 ]
 thirdparty_sources = [thirdparty_dir + file for file in thirdparty_sources]
 
 env_etcpak.Prepend(CPPPATH=[thirdparty_dir])
 
-# Also requires libpng headers
-if env["builtin_libpng"]:
-    env_etcpak.Prepend(CPPPATH=["#thirdparty/libpng"])
-
 env_thirdparty = env_etcpak.Clone()
 env_thirdparty.disable_warnings()
 env_thirdparty.add_source_files(thirdparty_obj, thirdparty_sources)