backend:/texture: NewTextureFromRgba causes potential crash

[gucio321]

Turns out that the following line:

        // Set finalizer
        runtime.SetFinalizer(&texture, (*Texture).release)

from backend/texture.go, which was intended to smoothly release C memory of freed by GC texture, in fact was a time bomb. When GC got the pointer it caused a SEGFAULT.

This is mmost likely because gl functions should be called in a very particular time (probably after or before ImGuiImpl_OpenGL3_NewFrame()) or maybe in a specific thred (?)