UTextureRenderTarget2D uses TC_Default for PF_R16F instead of TC_HalfFloat

When using "ConstructTexture2D" from a render target that uses the format PF_R16F (RTF_R16F) the output texture won't retain the format and will become a four channel float texture.  The texture format can be changed in the PreEditChange/PostEditChange scope, but it's more efficient to set it correctly in the first place and not cause the texture to be built again.

1. Created a UTextureRenderTarget2D with format RTF_R16F
2. Use the ConstructTexture2D function

Expected

The resulting new texture has the PF_R16F (TC_Half) format

Actual

The resulting texture is a float 4 (TC_Default) format