Arithemic operators added for ImVec2

[therealjoker4u]

Instead of doing this:

ImVec2 vec3 = ImVec2(vec1.x + vec2.x, vec1.y + vec2.y);
ImVec2 vec3 = ImVec2(vec1.x - vec2.x, vec1.y - vec2.y);
ImVec2 vec3 = ImVec2(vec1.x * vec2.x, vec1.y * vec2.y);
ImVec2 vec3 = ImVec2(vec1.x / vec2.x, vec1.y / vec2.y);
/// OR ->>>
ImVec2 vec3 = ImVec2(vec1.x + 10, vec1.y + 10);
ImVec2 vec3 = ImVec2(vec1.x - 10, vec1.y - 10);
ImVec2 vec3 = ImVec2(vec1.x * 10, vec1.y * 10);
ImVec2 vec3 = ImVec2(vec1.x / 10, vec1.y / 10);

Use the operators alternative:

ImVec2 vec3 = vec1 + vec2;
ImVec2 vec3 = vec1 - vec2;
ImVec2 vec3 = vec1 * vec2;
ImVec2 vec3 = vec1 / vec2;
/// OR ->>>
ImVec2 vec3 = vec1 + 10;
ImVec2 vec3 = vec1 - 10;
ImVec2 vec3 = vec1 * 10;
ImVec2 vec3 = vec1 / 10;

[ocornut]

Duplicate of #5966