stb_image_write.h: Consistently use STBIWDEF for stbi_write_*

Some functions were missing that in the definition, others weren't,
all had it in the declarations.

Added mention of JPG and HDR formats at the top of the file

stb_image_write.h

@@ -1,5 +1,5 @@
 /* stb_image_write - v1.05 - public domain - http://nothings.org/stb/stb_image_write.h
-   writes out PNG/BMP/TGA images to C stdio - Sean Barrett 2010-2015
+   writes out PNG/BMP/TGA/JPEG/HDR images to C stdio - Sean Barrett 2010-2015
                                      no warranty implied; use at your own risk
 
    Before #including,
@@ -466,7 +466,7 @@ static int stbi_write_tga_core(stbi__write_context *s, int x, int y, int comp, v
    return 1;
 }
 
-int stbi_write_tga_to_func(stbi_write_func *func, void *context, int x, int y, int comp, const void *data)
+STBIWDEF int stbi_write_tga_to_func(stbi_write_func *func, void *context, int x, int y, int comp, const void *data)
 {
    stbi__write_context s;
    stbi__start_write_callbacks(&s, func, context);
@@ -474,7 +474,7 @@ int stbi_write_tga_to_func(stbi_write_func *func, void *context, int x, int y, i
 }
 
 #ifndef STBI_WRITE_NO_STDIO
-int stbi_write_tga(char const *filename, int x, int y, int comp, const void *data)
+STBIWDEF int stbi_write_tga(char const *filename, int x, int y, int comp, const void *data)
 {
    stbi__write_context s;
    if (stbi__start_write_file(&s,filename)) {
@@ -636,7 +636,7 @@ static int stbi_write_hdr_core(stbi__write_context *s, int x, int y, int comp, f
    }
 }
 
-int stbi_write_hdr_to_func(stbi_write_func *func, void *context, int x, int y, int comp, const float *data)
+STBIWDEF int stbi_write_hdr_to_func(stbi_write_func *func, void *context, int x, int y, int comp, const float *data)
 {
    stbi__write_context s;
    stbi__start_write_callbacks(&s, func, context);
@@ -644,7 +644,7 @@ int stbi_write_hdr_to_func(stbi_write_func *func, void *context, int x, int y, i
 }
 
 #ifndef STBI_WRITE_NO_STDIO
-int stbi_write_hdr(char const *filename, int x, int y, int comp, const float *data)
+STBIWDEF int stbi_write_hdr(char const *filename, int x, int y, int comp, const float *data)
 {
    stbi__write_context s;
    if (stbi__start_write_file(&s,filename)) {