[feat] Acceleration and Flipping Gravity

[blunt-axe]

New keys are added:
[x]: The ball rotation has 1.5x acceleration when holding
[c]: Flip the ball's gravity

[blunt-axe]

However, my implementation for flipping gravity has some limitations.

The "glfwGetKey" function is rarely consecutive when holding a key. So, for every rigid body (src/GameX/physics/rigid_body.h), I choose to ignore the "flip gravity" signals which are too close to the last signal (precisely, less than 1/8 seconds). And I added a variable in every rigid body, representing the time lapsed since the last "flip gravity" input.

If there are better implementations, you may replace this one.

[blunt-axe]

However, my implementation for flipping gravity has some limitations.

The "glfwGetKey" function is rarely consecutive when holding a key. So, for every rigid body (src/GameX/physics/rigid_body.h), I choose to ignore the "flip gravity" signals which are too close to the last signal (precisely, less than 1/8 seconds). And I added a variable in every rigid body, representing the time lapsed since the last "flip gravity" input.

If there are better implementations, you may replace this one.