Adjust buffers usage

- Removed DrawQuad() function
- DrawBillboard() uses DrawBillboardRec()
- DrawPlane() uses RL_TRIANGLES
- DrawRectangleV() uses RL_TRIANGLES, that way, [shapes] module uses
only TRIANGLES buffers.

src/raylib.h

@@ -740,7 +740,6 @@ void DrawSphereWires(Vector3 centerPos, float radius, int rings, int slices, Col
 void DrawCylinder(Vector3 position, float radiusTop, float radiusBottom, float height, int slices, Color color); // Draw a cylinder/cone
 void DrawCylinderWires(Vector3 position, float radiusTop, float radiusBottom, float height, int slices, Color color); // Draw a cylinder/cone wires
 void DrawPlane(Vector3 centerPos, Vector2 size, Color color);                                      // Draw a plane XZ
-void DrawQuad(Vector3 v1, Vector3 v2, Vector3 v3, Vector3 v4, Color color);                        // Draw a quad
 void DrawRay(Ray ray, Color color);                                                                // Draw a ray line
 void DrawGrid(int slices, float spacing);                                                          // Draw a grid (centered at (0, 0, 0))
 void DrawGizmo(Vector3 position);                                                                  // Draw simple gizmo