Remove top-level const from function-scope variables

As per the const correctness rules, top-level const-ness of data
in automatic scopes does not propagate outside of these scopes
(unlike const-ness at lower levels, such as pointers to const data).

Previously, const was used liberally, but inconsistently across the
libpng codebase. Using const wherever applicable is not incorrect.
However, _consistent_ use of const is difficult to maintain in such
conditions.

In conclusion, we shall continue to use const only where doing so is
strictly necessary:

1. If a function guarantees that it will not modify an argument
   passed by pointer, the corresponding function parameter should be
   a pointer-to-const (const T *).

2. Static data should not be modified, therefore it should be const.

Reference:
Google C++ Style Guide
https://google.github.io/styleguide/cppguide.html#Use_of_const

pngset.c

@@ -137,7 +137,7 @@ png_set_cHRM_XYZ(png_const_structrp png_ptr, png_inforp info_ptr, double red_X,
 #ifdef PNG_eXIf_SUPPORTED
 void PNGAPI
 png_set_eXIf(png_const_structrp png_ptr, png_inforp info_ptr,
-    const png_bytep eXIf_buf)
+    png_bytep eXIf_buf)
 {
   png_warning(png_ptr, "png_set_eXIf does not work; use png_set_eXIf_1");
   PNG_UNUSED(info_ptr)
@@ -146,7 +146,7 @@ png_set_eXIf(png_const_structrp png_ptr, png_inforp info_ptr,
 
 void PNGAPI
 png_set_eXIf_1(png_const_structrp png_ptr, png_inforp info_ptr,
-    const png_uint_32 num_exif, const png_bytep eXIf_buf)
+    png_uint_32 num_exif, png_bytep eXIf_buf)
 {
    int i;