image: fix the boundary check in image_{hdc,jpg}
filelist_view: make the default _folder_actions const
dc_trans: don't modify rtgui_empty_point
dc_trans: move dx/dy out of loog and add some comment
dc_trans: add more const qualifier
dc_blend: no need to swap x/y in _draw_aa_line

It messup the lines.
dc_buffer: fix the rect check
dc_trans: small optimize
matrix: add some comment