Auto merge of #24055 - mmatyas:fix_webgl2_exts, r=jdm

Fix extension querying when using WebGL2

This patch fixes a crash caused by using a deprecated GL call. Starting with OpenGL 3 (used by WebGL2), the glGetString(GL_EXTENSIONS) call is deprecated, and some drivers produce GL_INVALID_ENUM error. Querying can be done by checking the number of extensions first, then getting the extensions one by one.

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