Fix scrollbar wrongly displayed bug

[tompng]

Fixes these scrollbar bugs

• Scrollbar is not shown at bottom when I scrolled to bottom
• Pink color is remained in scrollbar area
• Scrollbar sometimes disappears after you scroll or when dialog size change (not shown in the gif below)
• Scrollbar will disappear if contents is very long (not shown in the gif below)
  

Change 1

         pointer = dialog.pointer - dialog.scroll_top
+      else
+        dialog.scroll_top = 0
       end

Resets scroll_top to zero if dialog.pointer == nil. It will be nil when dialog_render_info.contents changed.
Originally, reset is done at dialog.scroll_top = dialog.contents.size - height that I deleted in change2.

Change 2

-    if dialog.contents and dialog.scroll_top >= dialog.contents.size
-      dialog.scroll_top = dialog.contents.size - height
-    end

The original code unintentionally resets dialog.scroll_top to zero because dialog.contents.size - height is always zero.
This will wrongly calculates scroll_top when scroll_top >= height.
I think the purpose of the original code is to satisfy the condition 0 <= scroll_top and scroll_top + height <= dialog_render_info.contents.size when the content shrinks.
After I add dialog.scroll_top = 0 in change1, the condition seems to be always satisfied.

Change 3

+      bar_height = 1 if bar_height.zero?

If the dialog content is too long, calculated bar_height will be zero.
Since most scrollbar in GUI apps has a minimum hight, I set the minimum hight to 1.

Change 4

-      if dialog.scrollbar_pos and (dialog.scrollbar_pos != old_dialog.scrollbar_pos or dialog.column != old_dialog.column)
+      if dialog.scrollbar_pos

The condition I deleted is for optimization to skip re-rendering scrollbar.
When pointer moved or dialog contents changed, scrollbar needs to be re-rendered. This is almost always, so I deleted the optimization condition.

[ima1zumi]

This 1 means minimum value. It is better to avoid magic numbers and use constants.

[tompng]

I Added MINIMUM_SCROLLBAR_HEIGHT = 1

[hasumikin]

LGTM!