plugins/screencast: Fix a glitch in cursor bitmap

In 52bc4606, some code was shuffled
around to improve code readability. However, it was overlooked that
spa_meta_bitmap->offset is initialized too late, after QImage dest is
constructed. That's the reason why the left edge of the cursor is
wrapped around horizontally.

This change fixes the cursor glitch by ensuring that
spa_meta_bitmap->offset is initialized to proper value before getting
pointer to bitmap data. While on this, this change also moves
spa_meta_bitmap initialization code around to make spa_meta_bitmap setup
look less like a bowl of spaghetti, i.e. spa_meta_bitmap is initialized
first, and QImage dest is created with spa_meta_bitmap's values.