godot: MeshLibraries don't preserve editor-created materials

Godot version:

3.0.6 (self-built from commit 8ac39d886307d76c286e804e027fc39f6b5aaac6) master (commit 8dd00ed1762c4956b3231d709ce0d01ee9b306c8)

OS/device including version:

Arch Linux (4.19.4-arch1-1-ARCH, Mesa 18.2.5-1)

Issue description:

Materials on editor-created meshes are not preserved in a MeshLibrary created via “Convert To -> MeshLibrary”. It sounds very similar to what was happening in #1239, but I’m not sure it’s the same issue.

Steps to reproduce:

1. Create scene with a root node and a child MeshInstance where mesh is created via “New PlaneMesh”
2. Create a SpatialMaterial for mesh, set Albedo to (255,0,0)
3. Save scene, then “Convert To” -> “MeshLibrary…”
4. Create GridMap in new scene, using the MeshLibrary created in Step 3 as theme.
5. Place the plane on grid

Expected: A red plane Result: A white plane

Minimal reproduction project: gridmap_material_issue_example.zip

** Additional Information: ** master build printed out the following after executing Step 3:

ERROR: create_from_image: Condition ' p_image.is_null() ' is true.
   At: scene/resources/texture.cpp:191.
ERROR: texture_get_data: Condition ' !texture->active ' is true. returned: Ref<Image>()
   At: drivers/gles3/rasterizer_storage_gles3.cpp:1033.
ERROR: _write_resource: Resource was not pre cached for the resource section, bug?
   At: scene/resources/scene_format_text.cpp:1369.

And the following after Step 4:

ERROR: create_from_image: Condition ' p_image.is_null() ' is true.
   At: scene/resources/texture.cpp:191.
ERROR: texture_get_data: Condition ' !texture->active ' is true. returned: Ref<Image>()
   At: drivers/gles3/rasterizer_storage_gles3.cpp:1033.
ERROR: texture_get_data: Condition ' !texture->active ' is true. returned: Ref<Image>()
   At: drivers/gles3/rasterizer_storage_gles3.cpp:1033.
ERROR: texture_get_data: Condition ' !texture->active ' is true. returned: Ref<Image>()
   At: drivers/gles3/rasterizer_storage_gles3.cpp:1033.
ERROR: texture_get_data: Condition ' !texture->active ' is true. returned: Ref<Image>()
   At: drivers/gles3/rasterizer_storage_gles3.cpp:1033.
ERROR: texture_get_data: Condition ' !texture->active ' is true. returned: Ref<Image>()
   At: drivers/gles3/rasterizer_storage_gles3.cpp:1033.
ERROR: texture_get_data: Condition ' !texture->active ' is true. returned: Ref<Image>()
   At: drivers/gles3/rasterizer_storage_gles3.cpp:1033.
ERROR: texture_get_data: Condition ' !texture->active ' is true. returned: Ref<Image>()
   At: drivers/gles3/rasterizer_storage_gles3.cpp:1033.
ERROR: texture_get_data: Condition ' !texture->active ' is true. returned: Ref<Image>()
   At: drivers/gles3/rasterizer_storage_gles3.cpp:1033.
ERROR: texture_get_data: Condition ' !texture->active ' is true. returned: Ref<Image>()
   At: drivers/gles3/rasterizer_storage_gles3.cpp:1033.
ERROR: texture_get_data: Condition ' !texture->active ' is true. returned: Ref<Image>()
   At: drivers/gles3/rasterizer_storage_gles3.cpp:1033.