Fix leaking environment RIDs in GLES2

[lawnjelly]

This PR adds the code necessary to correctly free environments in GLES2, which was previously missing causing them to leak.

Notes

• This was showing up in the tracked_handles build ([3.x] Add option to use handles to RID #54907)
• On tracking this down, it turned out although the client code was correctly calling free, the freeing code in RasterizerScene had been forgotten
• These will have been leaking during the editor and particularly during gameplay with multiple levels

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 3.4.1.