add -version option to anim_dump,anim_diff and img2webp

This is to harmonize the -h/-version options on all our examples.

+ added GetAnimatedImageVersions() method to anim_util.*

Change-Id: I2304a1c29e310682e97f236d3867274a192a7a09

examples/anim_util.h

@@ -56,6 +56,10 @@ void GetDiffAndPSNR(const uint8_t rgba1[], const uint8_t rgba2[],
                     uint32_t width, uint32_t height, int premultiply,
                     int* const max_diff, double* const psnr);
 
+// Return library versions used by anim_util.
+void GetAnimatedImageVersions(int* const decoder_version,
+                              int* const demux_version);
+
 #ifdef __cplusplus
 }    // extern "C"
 #endif