Port member initialization from constructor to declaration (C++11)

Using `clang-tidy`'s `modernize-use-default-member-init` check and
manual review of the changes, and some extra manual changes that
`clang-tidy` failed to do.

Also went manually through all of `core` to find occurrences that
`clang-tidy` couldn't handle, especially all initializations done
in a constructor without using initializer lists.

core/io/file_access_compressed.cpp

@@ -389,25 +389,6 @@ Error FileAccessCompressed::_set_unix_permissions(const String &p_file, uint32_t
 	return FAILED;
 }
 
-FileAccessCompressed::FileAccessCompressed() :
-		cmode(Compression::MODE_ZSTD),
-		writing(false),
-		write_ptr(nullptr),
-		write_buffer_size(0),
-		write_max(0),
-		block_size(0),
-		read_eof(false),
-		at_end(false),
-		read_ptr(nullptr),
-		read_block(0),
-		read_block_count(0),
-		read_block_size(0),
-		read_pos(0),
-		read_total(0),
-		magic("GCMP"),
-		f(nullptr) {
-}
-
 FileAccessCompressed::~FileAccessCompressed() {
 
 	if (f)