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 :)

thirdparty/pvrtccompressor/AlphaBitmap.h

@@ -0,0 +1,18 @@
+#pragma once
+
+#include "Bitmap.h"
+
+namespace Javelin {
+
+class AlphaBitmap : public Bitmap {
+public:
+    AlphaBitmap(int w, int h)
+        : Bitmap(w, h, 1) {
+    }
+
+    const unsigned char *GetData() const { return data; }
+
+    unsigned char *GetData() { return data; }
+};
+
+}