Stabilize C++ line by line rendering in the software renderer

[ogoffart]

The Line by Line rendering in C++ is gated by SLINT_FEATURE_EXPERIMENTAL
We should polish the API and make it available.
(Including in esp-idf)

[ogoffart]

The API we are talking about is

slint/api/cpp/include/slint-platform.h

Lines 696 to 716 in ac7c2dd

	# ifdef SLINT_FEATURE_EXPERIMENTAL
	/// Render the window scene, line by line. The provided Callback will be invoked for each line
	/// that needs to rendered.
	///
	/// The renderer uses a cache internally and will only render the part of the window
	/// which are dirty.
	///
	/// This function returns the physical region that was rendered considering the rotation.
	///
	/// The callback must be an invocable with the signature (size_t line, size_t begin, size_t end,
	/// auto render_fn). It is invoked with the line number as first parameter, and the start x and
	/// end x coordinates of the line as second and third parameter. The implementation must provide
	/// a line buffer (as std::span) and invoke the provided fourth parameter (render_fn) with it,
	/// to fill it with pixels. After the line buffer is filled with pixels, your implementation is
	/// free to flush that line to the screen for display.
	template<typename Callback>
	requires requires(Callback callback) {
	callback(size_t(0), size_t(0), size_t(0), [&callback](std::span<Rgb565Pixel>) {});
	}
	PhysicalRegion render_by_line(Callback process_line_callback) const
	{

And in esp-idf

slint/api/cpp/esp-idf/slint/include/slint-esp.h

Lines 22 to 26 in ac7c2dd

	* * Line-by-line rendering: Set neither `buffer1` nor `buffer2` to instruct Slint to allocate
	* a buffer (with MALLOC_CAP_INTERNAL) big enough to hold one line,
	* render into it, and send it to the display. This is an experimental
	* feature that requires enabling the `SLINT_FEATURE_EXPERIMENTAL` CMake
	* option.