libvpx: Pull from upstream

source/libvpx/vp9/common/vp9_convolve.c

Index: source/libvpx/vp9/common/vp9_convolve.c
===================================================================
--- source/libvpx/vp9/common/vp9_convolve.c	(revision 281795)
+++ source/libvpx/vp9/common/vp9_convolve.c	(working copy)
@@ -117,17 +117,25 @@
                      const InterpKernel *const y_filters,
                      int y0_q4, int y_step_q4,
                      int w, int h) {
-  // Fixed size intermediate buffer places limits on parameters.
-  // Maximum intermediate_height is 324, for y_step_q4 == 80,
-  // h == 64, taps == 8.
-  // y_step_q4 of 80 allows for 1/10 scale for 5 layer svc
-  uint8_t temp[64 * 324];
+  // Note: Fixed size intermediate buffer, temp, places limits on parameters.
+  // 2d filtering proceeds in 2 steps:
+  //   (1) Interpolate horizontally into an intermediate buffer, temp.
+  //   (2) Interpolate temp vertically to derive the sub-pixel result.
+  // Deriving the maximum number of rows in the temp buffer (135):
+  // --Smallest scaling factor is x1/2 ==> y_step_q4 = 32 (Normative).
+  // --Largest block size is 64x64 pixels.
+  // --64 rows in the downscaled frame span a distance of (64 - 1) * 32 in the
+  //   original frame (in 1/16th pixel units).
+  // --Must round-up because block may be located at sub-pixel position.
+  // --Require an additional SUBPEL_TAPS rows for the 8-tap filter tails.
+  // --((64 - 1) * 32 + 15) >> 4 + 8 = 135.
+  uint8_t temp[135 * 64];
   int intermediate_height = (((h - 1) * y_step_q4 + 15) >> 4) + SUBPEL_TAPS;
 
   assert(w <= 64);
   assert(h <= 64);
-  assert(y_step_q4 <= 80);
-  assert(x_step_q4 <= 80);
+  assert(y_step_q4 <= 32);
+  assert(x_step_q4 <= 32);
 
   if (intermediate_height < h)
     intermediate_height = h;