Fix frame duration calculation

[r-ralph]

Currently, rendering frame index is calculated from total duration of image. However, APNG can have duration for each frames (fcTL section).

The delay_num and delay_den parameters together specify a fraction indicating the time to display the current frame, in seconds. If the denominator is 0, it is to be treated as if it were 100 (that is, delay_num then specifies 1/100ths of a second). If the the value of the numerator is 0 the decoder should render the next frame as quickly as possible, though viewers may impose a reasonable lower bound.
Frame timings should be independent of the time required for decoding and display of each frame, so that animations will run at the same rate regardless of the performance of the decoder implementation.

https://wiki.mozilla.org/APNG_Specification#.60fcTL.60:_The_Frame_Control_Chunk