Set the USD schema multiccd scene flag default to True

[LouRohanNV]

multiccd was enabled by default for 3.8.0, but the USD schema was not updated. Ideally there should be a test to detect this mismatch.

[LouRohanNV]

I believe the proper test for this new change is to add these lines in mjc_physics_scene_test.cc around line 182:

  EXPECT_DISABLE_FLAG_USD_FALLBACK_EQ_MODEL_DEFAULT(IslandFlag, mjDSBL_ISLAND);
  EXPECT_DISABLE_FLAG_USD_FALLBACK_EQ_MODEL_DEFAULT(MultiCCDFlag,
                                                    mjDSBL_MULTICCD);

Also, I'm getting this from a review of the file:

The assertion line that was deleted in the "enable multiccd" commit was already broken on its own terms: the macro compared a bool to an int bitmask via EXPECT_EQ/EXPECT_NE, so 1 == (1<<19) is false for high bit positions. With mjENBL_MULTICCD = 1<<…, the assertion would never have actually matched the model default — it had to be replaced with the disable-flag form (mjDSBL_MULTICCD) anyway, but with the bool coercion fix it would have been a real check.

I'm not 100% certain if this is correct, but it suggests that these macros be adjusted to:

#define EXPECT_DISABLE_FLAG_USD_FALLBACK_EQ_MODEL_DEFAULT(usd_attr, mjc_flag) \
  {                                                                          \
    bool flag;                                                               \
    mjc_phys_scene.Get##usd_attr##Attr().Get(&flag);                         \
    EXPECT_NE(flag,                                                          \
              static_cast<bool>(default_model->opt.disableflags & (mjc_flag))); \
  }

#define EXPECT_ENABLE_FLAG_USD_FALLBACK_EQ_MODEL_DEFAULT(usd_attr, mjc_flag) \
  {                                                                          \
    bool flag;                                                               \
    mjc_phys_scene.Get##usd_attr##Attr().Get(&flag);                         \
    EXPECT_EQ(flag,                                                          \
              static_cast<bool>(default_model->opt.enableflags & (mjc_flag))); \
  }