all_zero cdf ctx: No need to clamp top and left

[wantehchang]

My colleague Vignesh Venkatasubramanian suggested this change.

In the procedure for computing the 'ctx' variable for the all_zero cdf, the following two lines can be omitted:

    top = Min( top, 255 )
    left = Min( left, 255 )

'top' and 'left' are the maximum of AboveLevelContext[ plane ][ x4 + k ] and LeftLevelContext[ plane ][ y4 + k ], respectively. The value we store in AboveLevelContext and LeftLevelContext are less than or equal to 63, because of the following line in the coeffs( plane, startX, startY, txSz ) function in Section 5.11.39:

    culLevel = Min( 63, culLevel )

[peterderivaz]

Good spot! This looks like a good simplification to me, but I don't know how much the working group want to modify the specification at this point.

I suggest you ask Adrian Grange if he wants it to be merged.