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/etc1/texture_loader_pkm.cpp

@@ -0,0 +1,84 @@
+#include "texture_loader_pkm.h"
+#include "os/file_access.h"
+#include <string.h>
+
+struct ETC1Header {
+    char tag[6];			// "PKM 10"
+    uint16_t format;        // Format == number of mips (== zero)
+    uint16_t texWidth;      // Texture dimensions, multiple of 4 (big-endian)
+    uint16_t texHeight;
+    uint16_t origWidth;     // Original dimensions (big-endian)
+    uint16_t origHeight;
+};
+
+RES ResourceFormatPKM::load(const String &p_path, const String& p_original_path, Error *r_error) {
+
+	if (r_error)
+		*r_error=ERR_CANT_OPEN;
+
+	Error err;
+	FileAccess *f = FileAccess::open(p_path,FileAccess::READ,&err);
+	if (!f)
+		return RES();
+
+	FileAccessRef fref(f);
+	if (r_error)
+		*r_error=ERR_FILE_CORRUPT;
+
+	ERR_EXPLAIN("Unable to open PKM texture file: "+p_path);
+	ERR_FAIL_COND_V(err!=OK,RES());
+
+	// big endian
+	f->set_endian_swap(true);
+
+	ETC1Header h;
+	ERR_EXPLAIN("Invalid or Unsupported PKM texture file: "+p_path);
+	f->get_buffer((uint8_t *) &h.tag, sizeof(h.tag));
+	if(strncmp(h.tag, "PKM 10", sizeof(h.tag)))
+		ERR_FAIL_V(RES());
+
+	h.format = f->get_16();
+	h.texWidth = f->get_16();
+	h.texHeight = f->get_16();
+	h.origWidth = f->get_16();
+	h.origHeight = f->get_16();
+
+	DVector<uint8_t> src_data;
+
+	uint32_t size = h.texWidth * h.texHeight / 2;
+	src_data.resize(size);
+	DVector<uint8_t>::Write wb = src_data.write();
+	f->get_buffer(wb.ptr(),size);
+	wb=DVector<uint8_t>::Write();
+
+	int mipmaps = h.format;
+	int width = h.origWidth;
+	int height = h.origHeight;
+
+	Image img(width,height,mipmaps,Image::FORMAT_ETC,src_data);
+
+	Ref<ImageTexture> texture = memnew( ImageTexture );
+	texture->create_from_image(img);
+
+	if (r_error)
+		*r_error=OK;
+
+	return texture;
+}
+
+void ResourceFormatPKM::get_recognized_extensions(List<String> *p_extensions) const {
+
+	p_extensions->push_back("pkm");
+}
+
+bool ResourceFormatPKM::handles_type(const String& p_type) const {
+
+	return ObjectTypeDB::is_type(p_type,"Texture");
+}
+
+String ResourceFormatPKM::get_resource_type(const String &p_path) const {
+
+	if (p_path.extension().to_lower()=="pkm")
+		return "ImageTexture";
+	return "";
+}