DARTHeightmapShape: correctly load asymmetric terrains

[scpeters]

Support for heightmaps in DART was added in bitbucket pull request 2956 and bitbucket pull request 3066 with tests using the heightmap_valley.png image, which happens to be symmetric in both x and y directions. I recently noticed a bug in DARTHeightmapShape when using asymmetric terrains, so I added a test using worlds/heightmap.world, which uses the asymmetric heightmap_bowl.png with a box at each of the 4 corners. The boxes all start at the same height, and based on the shape of the heightmap, 3 of the boxes should already be in contact with the heightmap, but one of them (box2) should fall. I've added a test that illlustrates the failure with dart:

[Dbg] [DARTPhysics.cc:734] Using BULLET collision detector
Dbg DART Bullet heightfield AABB: min = {-64.249, -64.249, -5}, max = {64.249, 64.249, 5} (will be translated by z=5)
AL lib: (EE) ALCpulsePlayback_streamStateCallback: Received stream failure!
[Dbg] [ServerFixture.cc:203] ServerFixture load in 0.8 seconds, timeout after 600 seconds
/data_fast/scpeters/ws/gazebo8/src/gazebo/test/integration/heightmap.cc:952: Failure
Expected: (box1->WorldPose().Pos().Z()) >= (9.9), actual: 5.0951 vs 9.9
/data_fast/scpeters/ws/gazebo8/src/gazebo/test/integration/heightmap.cc:956: Failure
Expected: (box2->WorldPose().Pos().Z()) < (5.5), actual: 9.98575 vs 5.5
[Dbg] [ServerFixture.cc:129] ServerFixture::Unload

The fix is to set an uninitialized variable DARTHeightmapShape::flipY to false.

Use gazebo --verbose worlds/heightmap.world -e dart to see the failure in gzclient.

[scpeters]

@osrf-jenkins run tests please

[scpeters]

@osrf-jenkins run tests please

[scpeters]

heightmap test is passing on Ubuntu and homebrew:

• https://build.osrfoundation.org/job/gazebo-ci-pr_any-ubuntu_auto-amd64-gpu-nvidia/333/
• https://build.osrfoundation.org/job/gazebo-ci-pr_any-homebrew-amd64/2107/

[claireyywang]

LGTM!

[scpeters]

thanks for the review!