Added contrib/pngminim/preader demo of gregbook/rpng2-x with embedded libpng

also minor editing of encoder/pngusr.h and decoder/pngusr.h

contrib/pngminim/decoder/pngusr.h

@@ -44,6 +44,7 @@
 #define PNG_NO_READ_sBIT
 #define PNG_NO_READ_sCAL
 #define PNG_NO_READ_sPLT
+#define PNG_NO_READ_sRGB
 #define PNG_NO_READ_TEXT
 #define PNG_NO_READ_tIME
 #define PNG_NO_READ_UNKNOWN_CHUNKS

contrib/pngminim/encoder/pngusr.h

@@ -33,7 +33,6 @@
 #define PNG_NO_WRITE_bKGD
 #define PNG_NO_WRITE_cHRM
 #define PNG_NO_WRITE_gAMA
-#define PNG_NO_WRITE_sRGB
 #define PNG_NO_WRITE_hIST
 #define PNG_NO_WRITE_iCCP
 #define PNG_NO_WRITE_oFFs
@@ -42,6 +41,7 @@
 #define PNG_NO_WRITE_sBIT
 #define PNG_NO_WRITE_sCAL
 #define PNG_NO_WRITE_sPLT
+#define PNG_NO_WRITE_sRGB
 #define PNG_NO_WRITE_TEXT
 #define PNG_NO_WRITE_tIME
 #define PNG_NO_WRITE_UNKNOWN_CHUNKS