Fix phpstan issues

[eigan]

The two most important issues affects projects using this library and also using phpstan. The error we get in our project is the following:

Call to method getImageAsString() on an unknown class static.

static is not in a namespace

-     * @return \static
+     * @return static

PHPStan thinks that \static is a class in the root namespace. Its perfectly fine to use just static instead.

Wrong return type

-     * @return integer
+     * @return float|integer

This method actually returns floats too. Not too important, but good to know.

Wrong casing of functions

-            $this->original_w = ImageSX($this->source_image);
-            $this->original_h = ImageSY($this->source_image);
+            $this->original_w = imagesx($this->source_image);
+            $this->original_h = imagesy($this->source_image);

Not affecting anything, so can revert this change if you like.

These changes will also make php-image-resize pass the first level (0)