Fixes the bug that framebuffer is not reset properly.

This commit fixes the `Widget::RenderDefaultFramebuffer()` method that the `default_framebuffer_` is not reset properly, and that can cause a problem when the size of the framebuffer changed. This commit also replaces `int` type with `float` type for variables keeping widget's dimension.
Adds a new translucent white highlighted style for button.

This commit implmenetes a new translucent white style for rendering button widget's default highlighted state. It works just like the translucent black style but with white color instead.