gbagfx: Various fixes and improvements

Fix num_colors exceeding amount of colors in palette leading to use of uninitialized colors

Handle png -> pal conversion (aka extracting JASC palettes from png

Allow using only file extension as output to reuse entire path from input before extension

tools/gbagfx/util.h

@@ -7,6 +7,7 @@
 
 bool ParseNumber(char *s, char **end, int radix, int *intValue);
 char *GetFileExtension(char *path);
+char *GetFileExtensionAfterDot(char *path);
 unsigned char *ReadWholeFile(char *path, int *size);
 unsigned char *ReadWholeFileZeroPadded(char *path, int *size, int padAmount);
 void WriteWholeFile(char *path, void *buffer, int bufferSize);