LibVideo/VP9: Implement unscaled fast paths in inter prediction

Inter-prediction convolution filters are selected based on the
subpixel position determined for the motion vector relative to the
block being predicted. The subpixel position 0 only uses one single
sample in the center of the convolution, not averaging any other
samples. Let's call this a copy.

Reference frames can also be a different size relative to the frame
being predicted, but in almost every case, that scale will be 1:1
for every single frame in a video.

Taking into account these facts, we can create multiple fast paths for
inter prediction. These fast paths are only active when scaling is 1:1.

If we are doing a copy in both dimensions, then we can do a straight
memcpy from the reference frame to the output block buffer. In videos
where there is no motion, this is a dramatic speedup.

If we are doing a copy in one dimension, we can just do one convolution
and average directly into the output block buffer.

If we aren't doing a copy in either dimension, we can still cut out a
few operations from the convolution loops, since we only need to
advance our samples by whole pixels instead of subpixels.

These fast paths result in about a 34% improvement (~31.2s -> ~20.6s)
in a video which relies heavily on intra-predicted blocks due to high
motion. In videos with less motion, the improvement will be even
greater.

Also, note that the accumulators in these faster loops are only 16-bit.
High bit-depth videos will overflow those, so for now the fast path is
only used for 8-bit videos.

Userland/Libraries/LibVideo/VP9/LookupTables.h

@@ -336,7 +336,7 @@ static constexpr u8 counter_to_context[19] = {
 };
 
 // Coefficients used by predict_inter
-static constexpr i32 subpel_filters[4][16][8] = {
+static constexpr i16 subpel_filters[4][16][8] = {
     { { 0, 0, 0, 128, 0, 0, 0, 0 },
         { 0, 1, -5, 126, 8, -3, 1, 0 },
         { -1, 3, -10, 122, 18, -6, 2, 0 },