[MC-133139] The image write callback is never freed, leaking small amounts of memory for each screenshot

Due to ~~MC-133136~~ it isn't possible to directly check this in current releases, but the same code is present still.

The STBIWriteCallback instance that is created to save screenshots is never free'd after use, causing a very small memory leak. (This callback exists to fix ~~MC-121714~~). Simply adding a free call or putting it into the existing try-with-resources block would solve this.

In 1.13-pre3, the leak detector (-Dorg.lwjgl.util.Debug=true -Dorg.lwjgl.util.DebugAllocator=true) reports:

[LWJGL] 16 bytes leaked, thread 1 (Client thread), address: 0x2110000
	at org.lwjgl.stb.STBIWriteCallback.<init>(STBIWriteCallback.java:37)
	at dfw$a.<init>(SourceFile:325)
	at dfw$a.<init>(SourceFile:321)
	at dfw.a(SourceFile:349)
	at dfw.a(SourceFile:318)
	at cjh.a(SourceFile:45)
	at cjh.a(SourceFile:30)
	at ciz.a(SourceFile:276)
	at org.lwjgl.glfw.GLFWKeyCallbackI.callback(GLFWKeyCallbackI.java:23)
	at org.lwjgl.system.JNI.invokeV(Native Method)
	at org.lwjgl.glfw.GLFW.glfwPollEvents(GLFW.java:2520)
	at cjm.a(SourceFile:296)
	at cjc.c(SourceFile:839)
	at cjc.a(SourceFile:378)
	at net.minecraft.client.main.Main.main(SourceFile:143)