Rescale the image for the rect in UIButton

[HantianZheng]

Is your feature request related to a problem? Please describe.
I think it will be very handy if the image(e.g. normal image, hover image) are auto-rescaled to the size of the rect in the UI Button.

Describe the solution you'd like

I have modified the code myself and I think this could be a good example.

in pygame_gui\elements\ui_button.py

`def _set_any_images_from_theme(self):

    changed = False
    normal_image = None
    try:
        normal_image = self.ui_theme.get_image('normal_image', self.combined_element_ids)
        normal_image = pygame.transform.scale(normal_image, self.relative_rect.size)  # auto-rescale the image
    except LookupError:
        normal_image = None
    finally:
        if normal_image != self.normal_image:
            self.normal_image = normal_image
            self.hovered_image = normal_image
            self.selected_image = normal_image
            self.disabled_image = normal_image
            changed = True

    hovered_image = None
    try:
        hovered_image = self.ui_theme.get_image('hovered_image', self.combined_element_ids)
        hovered_image = pygame.transform.scale(hovered_image, self.relative_rect.size)  # auto-rescale the image
    except LookupError:
        hovered_image = self.normal_image
    finally:
        if hovered_image != self.hovered_image:
            self.hovered_image = hovered_image
            changed = True

    selected_image = None
    try:
        selected_image = self.ui_theme.get_image('selected_image', self.combined_element_ids)
        selected_image = pygame.transform.scale(selected_image, self.relative_rect.size)  # auto-rescale the image
    except LookupError:
        selected_image = self.normal_image
    finally:
        if selected_image != self.selected_image:
            self.selected_image = selected_image
            changed = True

    disabled_image = None
    try:
        disabled_image = self.ui_theme.get_image('disabled_image', self.combined_element_ids)
        disabled_image = pygame.transform.scale(disabled_image, self.relative_rect.size)  # auto-rescale the image
    except LookupError:
        disabled_image = self.normal_image
    finally:
        if disabled_image != self.disabled_image:
            self.disabled_image = disabled_image
            changed = True

    return changed`