As `add_rect_filled_multicolor` does not have any option, the `build`
pattern is not used and calling `add_rect_filled_multicolor` directly
draws the rectangle on the window.
This patch uses bitflags to set the whether the corners are rounded.
Hence the `ImDrawCornerFlags` struct is defined, but only used
internally.
Externally, the valule of the flags can be changed with methods on the
`Rect` structure such as `round_top_right` or `round_bot_left`.
This patch wraps both ImDrawList_AddRectFilled and ImDrawList_AddRect.
ImDrawList_AddRectFilled is seen as a particular case of `add_rect`
where `filled` is set to `true`.
This patch makes the basic structure for a wrapper around Dear ImGui's
drawing API.
1. Implement `with_window_draw_list` method on Ui. Call this method to
get access to the `WindowDrawList` object. This object holds the methods
to access ImGui's drawing API.
2. Dear ImGui uses the ImU32 (an unsigned c_int) to represent colors in
the drawing API. This commit wraps this type with ImColor for
convenience. Any color representation (3or4-tuples, 3or4-arrays, ImU32
or ImVec4) can be converted into ImColor for convenience.
3. Difference between WindowDrawList and ChannelsSplit: Most drawing
functions can be called on WindowDrawList and ChannelsSplit objects.
However for safety, some functions can only be called on WindowDrawList
or ChannelsSplit instance. For example `channels_set_current` can only
be called after channels have been split. To avoid code duplication,
functions common to WindowDrawList and ChannelsSplit are implemented
within the `impl_draw_list_methods` macro.
4. Implement drawing functions (in this commit, add_line only). Calling
`add_line` returns a default representation of the line to be drawn, but
does not draw it. Then parameters, such as thickness, can be set. You
must call `build` to draw the line. All drawing functions will be
implemented following this pattern.
