Declare 'avif' param of avifJPEG/PNGWrite as const (#508)

Declare the 'avifImage * avif' parameters of avifJPEGWrite() and
avifPNGWrite() as const.

apps/shared/avifjpeg.c

diff --git a/apps/shared/avifjpeg.c b/apps/shared/avifjpeg.c
index 17c7469..208a7ca 100644
--- a/apps/shared/avifjpeg.c
+++ b/apps/shared/avifjpeg.c
@@ -112,7 +112,7 @@
     return ret;
 }
 
-avifBool avifJPEGWrite(const char * outputFilename, avifImage * avif, int jpegQuality, avifChromaUpsampling chromaUpsampling)
+avifBool avifJPEGWrite(const char * outputFilename, const avifImage * avif, int jpegQuality, avifChromaUpsampling chromaUpsampling)
 {
     avifBool ret = AVIF_FALSE;
     FILE * f = NULL;

apps/shared/avifjpeg.h

diff --git a/apps/shared/avifjpeg.h b/apps/shared/avifjpeg.h
index 38ca854..36df980 100644
--- a/apps/shared/avifjpeg.h
+++ b/apps/shared/avifjpeg.h
@@ -7,6 +7,6 @@
 #include "avif/avif.h"
 
 avifBool avifJPEGRead(const char * inputFilename, avifImage * avif, avifPixelFormat requestedFormat, uint32_t requestedDepth);
-avifBool avifJPEGWrite(const char * outputFilename, avifImage * avif, int jpegQuality, avifChromaUpsampling chromaUpsampling);
+avifBool avifJPEGWrite(const char * outputFilename, const avifImage * avif, int jpegQuality, avifChromaUpsampling chromaUpsampling);
 
 #endif // ifndef LIBAVIF_APPS_SHARED_AVIFJPEG_H

apps/shared/avifpng.c

diff --git a/apps/shared/avifpng.c b/apps/shared/avifpng.c
index e7450ab..7d5c5f4 100644
--- a/apps/shared/avifpng.c
+++ b/apps/shared/avifpng.c
@@ -155,7 +155,7 @@
     return readResult;
 }
 
-avifBool avifPNGWrite(const char * outputFilename, avifImage * avif, uint32_t requestedDepth, avifChromaUpsampling chromaUpsampling)
+avifBool avifPNGWrite(const char * outputFilename, const avifImage * avif, uint32_t requestedDepth, avifChromaUpsampling chromaUpsampling)
 {
     volatile avifBool writeResult = AVIF_FALSE;
     png_structp png = NULL;

apps/shared/avifpng.h

diff --git a/apps/shared/avifpng.h b/apps/shared/avifpng.h
index ed341f7..45c7f75 100644
--- a/apps/shared/avifpng.h
+++ b/apps/shared/avifpng.h
@@ -8,6 +8,6 @@
 
 // if (requestedDepth == 0), do best-fit
 avifBool avifPNGRead(const char * inputFilename, avifImage * avif, avifPixelFormat requestedFormat, uint32_t requestedDepth, uint32_t * outPNGDepth);
-avifBool avifPNGWrite(const char * outputFilename, avifImage * avif, uint32_t requestedDepth, avifChromaUpsampling chromaUpsampling);
+avifBool avifPNGWrite(const char * outputFilename, const avifImage * avif, uint32_t requestedDepth, avifChromaUpsampling chromaUpsampling);
 
 #endif // ifndef LIBAVIF_APPS_SHARED_AVIFPNG_H