Fix signed and unsigned comparisons

The first in my quest to make Godot 3.x compile with -Werror on GCC7

scene/resources/texture.cpp

@@ -501,9 +501,9 @@ Error StreamTexture::_load_data(const String &p_path, int &tw, int &th, int &fla
 		Vector<Ref<Image> > mipmap_images;
 		int total_size = 0;
 
-		for (int i = 0; i < mipmaps; i++) {
+		for (uint32_t i = 0; i < mipmaps; i++) {
 
-			if (i > 0) {
+			if (i) {
 				size = f->get_32();
 			}