New SDF objective function.

This solves the jittery behavior observed with the gear example in the case of small applied torques (~0.5).

The new quadratic option has the form max(A, 0)^2/2 + max(B, 0)^2/2 - min(A, 0)*min(B, 0). This function has a minimum in the intersections of two SDFs A and B, while avoiding the flat areas which would be generated if only the clearance field A+B were employed. See for example [the function resulting from two colliding circles](https://www.wolframalpha.com/input?i=minimize+max%28sqrt%28x%5E2%2By%5E2%29-1%2C0%29%5E2+%2B+max%28sqrt%28%28x-1%29%5E2%2B%28y-1%29%5E2%29-1%2C0%29%5E2+-+2*min%28sqrt%28x%5E2%2By%5E2%29-1%2C0%29*min%28sqrt%28%28x-1%29%5E2%2B%28y-1%29%5E2%29-1%2C0%29)

PiperOrigin-RevId: 583992855
Change-Id: I135a1b5931cd136d7d33cc275f8d361a8b7e290c

src/engine/engine_collision_sdf.h

@@ -24,10 +24,11 @@
 extern "C" {
 #endif
 
-typedef enum mjtSDFType_ {
-  mjSDFTYPE_SINGLE     = 0,
-  mjSDFTYPE_INTERSECTION,
-  mjSDFTYPE_AVERAGE,
+typedef enum mjtSDFType_ {        // signed distance function (SDF) type
+  mjSDFTYPE_SINGLE     = 0,       // single SDF
+  mjSDFTYPE_INTERSECTION,         // max(A, B)
+  mjSDFTYPE_MIDSURFACE,           // A - B
+  mjSDFTYPE_QUADRATIC,            // max(A, 0)^2/2 + max(B, 0)^2/2 + min(A, 0)*min(B, 0)
 } mjtSDFType;
 
 struct mjSDF_ {