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/image.cpp

@@ -3671,14 +3671,3 @@ Ref<Resource> Image::duplicate(bool p_subresources) const {
 void Image::set_as_black() {
 	zeromem(data.ptrw(), data.size());
 }
-
-Image::Image() {
-
-	width = 0;
-	height = 0;
-	mipmaps = false;
-	format = FORMAT_L8;
-}
-
-Image::~Image() {
-}