stb_image: Fix name of stbi_set_unpremultiply_on_load_thread impl

Bug reported by N-R-K. Fixes #1276.

stb_image.h

@@ -4956,7 +4956,7 @@ STBIDEF void stbi_convert_iphone_png_to_rgb(int flag_true_if_should_convert)
 static STBI_THREAD_LOCAL int stbi__unpremultiply_on_load_local, stbi__unpremultiply_on_load_set;
 static STBI_THREAD_LOCAL int stbi__de_iphone_flag_local, stbi__de_iphone_flag_set;
 
-STBIDEF void stbi__unpremultiply_on_load_thread(int flag_true_if_should_unpremultiply)
+STBIDEF void stbi_set_unpremultiply_on_load_thread(int flag_true_if_should_unpremultiply)
 {
    stbi__unpremultiply_on_load_local = flag_true_if_should_unpremultiply;
    stbi__unpremultiply_on_load_set = 1;