GPUImmediate: Make activation / deactivation implicit

This avoids unecessary complexity.

Also makes the GPUImmediate threadsafe by using a threadlocal imm variable.

source/blender/gpu/GPU_immediate.h

@@ -145,12 +145,6 @@ void immUniformThemeColorBlendShade(int color_id1, int color_id2, float fac, int
 void immUniformThemeColorBlend(int color_id1, int color_id2, float fac);
 void immThemeColorShadeAlpha(int colorid, int coloffset, int alphaoffset);
 
-/* These are called by the system -- not part of drawing API. */
-void immInit(void);
-void immActivate(void);
-void immDeactivate(void);
-void immDestroy(void);
-
 #ifdef __cplusplus
 }
 #endif