render: convert tmotion vectors to render basis

[marcin-serwin]

Description

When using SDL_ConvertEventToRenderCoordinates with SDL_EVENT_FINGER_MOTION events it converts x and y coordinates but does not convert the the dx and the dy unlike xrel and yrel of mouse motion events. This is means that these are rather useless after conversion. This change unifies this behavior between touch and mouse motion events.

Existing Issue(s)

N/A