[FEATURE] mesh simplification (qgis#34532)

[FEATURE] fix qgis#31575 mesh layer speed rendering improvements (LOD)

As mesh layer could have millions of triangles, rendering could be very slow, especially when all the triangles are displayed in the view whereas triangles are too small to be viewed.

For those situations QGIS has an option to simplify the mesh. Simplification leads to one or more simplified mesh that represents levels of detail (LOD). When rendering the mesh, the appropriate level of detail is chosen to have an adequate rendering depending on the view.

Author: vcloarec

external/meshOptimizer/LICENSE.md

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+MIT License
+
+Copyright (c) 2016-2019 Arseny Kapoulkine
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

external/meshOptimizer/README.md

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+[meshOptimiser](https://github.com/zeux/meshoptimizer) v 0.13
+
+[meshOptimiser](https://github.com/zeux/meshoptimizer) is a library that provides algorithms to optimize meshes.
+One of those algorithms simplifies meshes, e.i., reduces the number of vertices/triangles of the mesh. This library is oriented GPU rendering, but it could be used for other meshes.
+
+In QGIS the [simplification algorithm](https://github.com/zeux/meshoptimizer#simplification) is used to simplify mesh layers to speed up rendering.
+
+As mesh layer could have millions of triangles, rendering could be very slow, especially when all the triangles are displayed in the view whereas triangles are too small to be viewed. For those situations, an option provides the possibility to simplify the mesh. Simplification leads to one or more simplified mesh that represents levels of detail. When rendering the mesh, the appropriate level of detail is chosen to have an adequate rendering depending on the view.
+