Lansdcape grass generation fails on Mac running OSX 10.10.1

Grass map heights all appear to be 0 when generating grass on a Mac running 10.10. This bug does not occur on Macs running OSX 10.9.5

Putting a breakpoint in LandscapeGrass.cpp line 476 you can see that while weight data is returned in the B channel, the height data in R and G are always 0.

Editing LandscapeGrassWeight.usf and hardcoding the height values in the pixel shader makes it back to cpp. Hardcoding Output.Height value in the vertex shader before it's interpolated to the pixel shader does not work, so the values are not being interpolated for some reason.

1. Open QAGame editor and load the map Developers\jack.porter\GrassRtoT\GrassSubsections.umap
2. run the console command grass.flushcache

On PC and 10.9.5, the grass is regenerated. On 10.10.3 it is generated below the landscape.