dds/etc1/pbm/pvr: Make those modules and split thirdparty files

They are not particularly packaged in Linux distros so we do not
facilitate unbundling via SCons. There could be done if/when there
is interest.

Also s/pnm/pbm/, long-lived typo :)

modules/pvr/SCsub

@@ -0,0 +1,23 @@
+Import('env')
+Import('env_modules')
+
+# Thirdparty source files
+# Not unbundled so far since not widespread as shared library
+thirdparty_dir = "#thirdparty/pvrtccompressor/"
+thirdparty_pvr_sources = [
+	"BitScale.cpp",
+	"MortonTable.cpp",
+	"PvrTcDecoder.cpp",
+	"PvrTcEncoder.cpp",
+	"PvrTcPacket.cpp",
+]
+thirdparty_pvr_sources = [thirdparty_dir + file for file in thirdparty_pvr_sources]
+
+env_modules.add_source_files(env.modules_sources, thirdparty_pvr_sources)
+env_modules.Append(CPPPATH = [thirdparty_dir])
+
+# Godot source files
+env_modules.add_source_files(env.modules_sources, "*.cpp")
+
+Export('env_modules')
+Export('env')