fill_sub_image for textures

[nowakf]

Filling a subset of a texture is sometimes useful, and would basically require something like:

    ///
    /// Fills a region of this texture with the given data and generate mip maps if specified at construction.
    ///
    /// # Panic
    /// Will panic if the length of the data does not correspond to the width, height and format specified at construction.
    /// Will panic if the specified region is out of bounds.
    /// It is therefore necessary to create a new texture if the texture size or format has changed.
    ///
    pub fn fill_subset<T: TextureDataType>(&self, x: u32, y: u32, w: u32, h: u32, data: &[T]) {
        check_data_length::<T>(w, h, 1, self.data_byte_size, data.len());
        assert!(x+w < self.width && y+h < self.height);
        self.bind();
        let mut data = data.to_owned();
        flip_y(&mut data, self.width as usize, self.height as usize);
        unsafe {
            self.context.tex_sub_image_2d(
                crate::context::TEXTURE_2D,
                0,
                x as i32,
                y as i32,
                w as i32,
                h as i32,
                format_from_data_type::<T>(),
                T::data_type(),
                crate::context::PixelUnpackData::Slice(Some(to_byte_slice(&data))),
            );
        }
        self.generate_mip_maps();
    }

--- to be added to all the various Texture types.

What do you think?