GLES2 Lightmap baking issue with GridMap

[vansoest]

Godot version:
3.2.1, godot-bin packaged from Arch User Repository.

OS/device including version:
Arch Linux latest 2020-04-06
Kernel version: 5.5.13-arch2-1
NVidia module driver version: 440.64
CPU: Intel(R) Core(TM) i7-4790K CPU @ 4.00GHz
GPU: 01:00.0 VGA compatible controller: NVIDIA Corporation GP107 [GeForce GTX 1050 Ti] (rev a1)

Issue description:
Got Artifacts by baking the Lightmaps for GLES2 with a GridMap.
screenshot.png

Steps to reproduce:
Import the glTF 2.0 file into Godot as scene with Light Baking: Gen Lightmaps as import setting to auto unwrap uv2 maps. Export the Scene as tiles.meshlib.

Add following Nodes:

• Add WorldEnvironment and Environment with ProceduralSky.
• Add GridMap with tiles.meshlib.
• Add DirectionalLight and transform the rotation.
• Add BakedLightmap change Mode to RayTrace and Quality to High.

Draw a test case in the GridMap and change the extends of the BakedLightmap suitable for the test case.
Click on Bake Lightmaps button.

Minimal reproduction project:
https://github.com/vansoest/GLES2LightmapIssue

[Calinou]

Duplicate of #29203 and #30929. The lightmapper will be rewritten in Godot 4.0.