Implement control: GuiScrollPanel()

[raysan5]

It would be very useful to have a panel with scrolling that sizes with its contents and offers scroll bars. This is the proposed implementation:

// Scroll Panel control
Vector2 GuiScrollPanel(Rectangle bounds, Rectangle content, Vector2 viewScroll);

Rectangle bounds define the visible area
Rectangle content defines size of internal data and will condition scrolling
Vector2 viewScroll defines the content offset, new offset should be returned in case of scrolling

Note that content.x and content.y could be useless... or just use them as scroll offset.

Check GuiListView() for a temptative implementation of GuiScrollPanel().

[raysan5]

Partially implemented in commit ef9eb88.