Refactor: is_exr_file checks memory buffer

All places that used is_exr_file already had first four bytes
of the file read into memory. So make is_exr_file work just
like is_dds_file does and check those bytes directly, no need
to hit stdio again.

renderdoc/common/dds_readwrite.cpp

@@ -952,7 +952,7 @@ RDResult write_dds_to_file(FILE *f, const write_tex_data &data)
   return RDResult();
 }
 
-bool is_dds_file(byte *headerBuffer, size_t size)
+bool is_dds_file(const byte *headerBuffer, size_t size)
 {
   if(size < 4)
   {

renderdoc/common/dds_readwrite.h

@@ -28,6 +28,6 @@
 #include "serialise/streamio.h"
 #include "tex_data.h"
 
-extern bool is_dds_file(byte *headerBuffer, size_t size);
+extern bool is_dds_file(const byte *headerBuffer, size_t size);
 extern RDResult load_dds_from_file(StreamReader *reader, read_tex_data &data);
 extern RDResult write_dds_to_file(FILE *f, const write_tex_data &data);