Rotating text and icon demo for dear imgui

ImRotateDemo.cpp

#include "imgui_internal.h"

int rotation_start_index;

auto ImRotateStart() -> void {
	rotation_start_index = ImGui::GetWindowDrawList()->VtxBuffer.Size;
}

auto ImRotationCenter() -> ImVec2 {
	ImVec2 l{FLT_MAX, FLT_MAX}, u{-FLT_MAX, -FLT_MAX}; // bounds

	const auto& buf = ImGui::GetWindowDrawList()->VtxBuffer;
	for (int i = rotation_start_index; i < buf.Size; i++)
		l = ImMin(l, buf[i].pos), u = ImMax(u, buf[i].pos);

	return {(l.x + u.x) / 2, (l.y + u.y) / 2}; // or use _ClipRectStack?
}

void ImRotateEnd(float rad, ImVec2 center = ImRotationCenter()) {
	float s = ImSin(rad), c = ImCos(rad);
	center = ImRotate(center, s, c) - center;

	auto& buf = ImGui::GetWindowDrawList()->VtxBuffer;
	for (int i = rotation_start_index; i < buf.Size; i++)
		buf[i].pos = ImRotate(buf[i].pos, s, c) - center;
}