8 uvs for glTF2, URI decode and Vertex Custom api

[fire]

Fixes: #44552
Fixes: godotengine/godot-proposals#2266

Based upon the work of thenegativehunter2.

Use https://github.com/KhronosGroup/glTF-Sample-Models/tree/master/2.0/MultiUVTest to test.

Use for import tests:
gltf_test_8_uvs.zip

Use for export tests:

https://github.com/thenegativehunter2/godot_custom_vertex_attribute_demo/blob/master/assets/meshes/DualSurfaceMesh.mesh:

gltf_uvbug01glb.zip

[fire]

Current status:

• Exporting a godot scene to gltf2 seems to export texcoord0 to texcoord7
• Import gltf2 with texcoord0 to texcoord7 doesn't seem to work
• customs are 4 floats and not colors.
• Write a test case project in Godot Engine

[lyuma]

Updated the review with support for float based custom formats in SurfaceTool, EditorSceneImporterMesh and GLTFDocument.

Future work: support importing uvs to half-based or unorm-based CUSTOM* channels; support half and unorm/snorm types in SurfaceTool::create_vertex_array_from_triangle_arrays and support exporting half and unorm/snorm types to TEXCOORD_ channels in gltf.

Here's a shader program I use to test UVs. It shows 3 of the 4 channels based on screen quadrant:
UV/UV2 in left 25%, CUSTOM0 in 25%-50%, CUSTOM1 in 50%-75% and CUSTOM2 in 75%-100%

test_8_uvs.gdshader

It does seem like CUSTOM0, CUSTOM1 and CUSTOM2 show the same data in the shader. I tried reordering the bufferviews such that it should have guaranteed the custom channels are different, but I haven't entirely ruled out yet if the input data is bad.
Anyway this is a good start. We'll need to do more extensive testing.

[fire]

Demo Project and video.

2021-09-08_23-05-35.mp4

multi_uv_set_test.zip

[reduz]

Looks good to me, will merge later today or tomorrow if no one else objects.