Fix some easing equations' undefined behaviours.
Fix some easing equations' undefined behaviours.

Spotted via -Wunsequenced.
Easing equations had different behaviours depending on the toolchain
due to its abusing of inline assignments.

Valentactive/godot,Shockblast/godot,josempans/godot,Zylann/godot,MarianoGnu/godot,akien-mga/godot,godotengine/godot,MarianoGnu/godot,vkbsb/godot,Zylann/godot,Valentactive/godot,godotengine/godot,Shockblast/godot,honix/godot,josempans/godot,Faless/godot,MarianoGnu/godot,firefly2442/godot,vnen/godot,vnen/godot,Zylann/godot,Faless/godot,Valentactive/godot,MarianoGnu/godot,Faless/godot,BastiaanOlij/godot,Valentactive/godot,sanikoyes/godot,vnen/godot,akien-mga/godot,Shockblast/godot,Zylann/godot,josempans/godot,guilhermefelipecgs/godot,firefly2442/godot,godotengine/godot,ZuBsPaCe/godot,josempans/godot,ZuBsPaCe/godot,guilhermefelipecgs/godot,josempans/godot,firefly2442/godot,guilhermefelipecgs/godot,Faless/godot,ZuBsPaCe/godot,Faless/godot,firefly2442/godot,honix/godot,sanikoyes/godot,Shockblast/godot,sanikoyes/godot,pkowal1982/godot,akien-mga/godot,pkowal1982/godot,akien-mga/godot,DmitriySalnikov/godot,pkowal1982/godot,guilhermefelipecgs/godot,BastiaanOlij/godot,vkbsb/godot,Valentactive/godot,Faless/godot,MarianoGnu/godot,MarianoGnu/godot,MarianoGnu/godot,BastiaanOlij/godot,Shockblast/godot,firefly2442/godot,BastiaanOlij/godot,vkbsb/godot,DmitriySalnikov/godot,josempans/godot,josempans/godot,sanikoyes/godot,Valentactive/godot,vnen/godot,godotengine/godot,akien-mga/godot,guilhermefelipecgs/godot,josempans/godot,akien-mga/godot,firefly2442/godot,Shockblast/godot,godotengine/godot,guilhermefelipecgs/godot,BastiaanOlij/godot,ZuBsPaCe/godot,vkbsb/godot,pkowal1982/godot,Shockblast/godot,Faless/godot,ZuBsPaCe/godot,guilhermefelipecgs/godot,Valentactive/godot,Valentactive/godot,Shockblast/godot,honix/godot,Zylann/godot,BastiaanOlij/godot,akien-mga/godot,firefly2442/godot,firefly2442/godot,Zylann/godot,MarianoGnu/godot,DmitriySalnikov/godot,ZuBsPaCe/godot,vnen/godot,vkbsb/godot,DmitriySalnikov/godot,pkowal1982/godot,vnen/godot,pkowal1982/godot,godotengine/godot,sanikoyes/godot,Faless/godot,honix/godot,pkowal1982/godot,godotengine/godot,ZuBsPaCe/godot,vkbsb/godot,vkbsb/godot,sanikoyes/godot,BastiaanOlij/godot,DmitriySalnikov/godot,ZuBsPaCe/godot,guilhermefelipecgs/godot,honix/godot,DmitriySalnikov/godot,godotengine/godot,Zylann/godot,BastiaanOlij/godot,akien-mga/godot,sanikoyes/godot,sanikoyes/godot,vnen/godot,Zylann/godot,pkowal1982/godot,vnen/godot,DmitriySalnikov/godot,honix/godot,vkbsb/godot