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.