Bitmap class

[qnighy]

• [x] Bitmap.new
• [x] Bitmap#dispose, Bitmap#disposed?
• [x] Bitmap#width, Bitmap#height, Bitmap#rect
• [x] Bitmap#blt
• [x] Bitmap#stretch_blt
• [x] Bitmap#fill_rect
• [x] Bitmap#gradient_fill_rect
• [x] Bitmap#clear, Bitmap#clear_rect
• [x] Bitmap#get_pixel, Bitmap#set_pixel
• [x] Bitmap#hue_change
• [x] Bitmap#blur
• [x] Bitmap#radial_blur
• [x] Bitmap#draw_text
• [x] Bitmap#text_size
• [x] Bitmap#font, Bitmap#font=

[qnighy]

I'd like to mark the rest Bitmap methods (hue_change, blur, radial_blur) as 0.1.0 milestone.

[qnighy]

Done!