fix: TextMeshPro materials used by TextShape are not properly destroyed

[sandrade-dcl]

What does this PR change?

Fix #772

There were lots of mesh count related to TextMeshPro on unity's profiler.
We should analyze if the TextShape component is destroyed correctly

Implemented solution

1. Each time a TextShape object is created, we store a copy of the shared material in a private variable.
2. Instead of using text.fontMaterial..., we use text.fontSharedMaterial... in order to not to provoke new copies of the material.
3. Once the TextShape object is destroyed, we destroy our stored material.

NOTE: Making different tests with the TextShape components loading isolated, I have not seen a huge memory saving (the most important issue that I've detected with the number of materials in the profiler has been telated to this other ISSUE). Anyway, in order to avoid problems with the material handling, I think it is the best way to manage the cleaning of font materials.

Test link

https://play.decentraland.zone/index.html?renderer=urn:decentraland:off-chain:renderer-artifacts:fix/TextShape-memory-leaks&ENV=org

Our Code Review Standards

https://github.com/decentraland/unity-renderer/blob/master/docs/code-review-standards.md

[github-actions]

After the CI passes:

• This branch can be previewed at https://play.decentraland.zone/index.html?renderer=urn:decentraland:off-chain:renderer-artifacts:fix/TextShape-memory-leaks
• Code coverage report: https://renderer-artifacts.decentraland.org/branch-coverage/fix/TextShape-memory-leaks

[BrianAmadori]

LGTM