Investigate memory leak in rasterMathText with OpenGL backend

[t20100]

The change of text rasterization (4047) has created issues in flint (https://gitlab.esrf.fr/bliss/bliss/-/issues/4539) and has been reverted in PR #4186 to use Qt instead of matplotlib whenever possible and released as v2.1.2 to mitigate this issue. The root cause is not identified and more investigation is needed to try and find a fix or port PR #4186 to the main branch.