intel: Add assert check for blitting alignment.

Also fixup code comment to reflect that the GPU requires DWORD
alignment, but in this case does not actually pass the value "in
DWORDs" as I previously stated.

src/mesa/drivers/dri/intel/intel_blit.c

@@ -483,8 +483,8 @@ intel_emit_linear_blit(struct intel_context *intel,
    /* Blits are in a different ringbuffer so we don't use them. */
    assert(intel->gen < 6);
 
-   /* The pitch hits the GPU as a is a signed value, IN DWORDs.
-    * But we want width to match pitch. Max width is (1 << 15 - 1),
+   /* The pitch given to the GPU must be DWORD aligned, and
+    * we want width to match pitch. Max width is (1 << 15 - 1),
     * rounding that down to the nearest DWORD is 1 << 15 - 4
     */
    pitch = MIN2(size, (1 << 15) - 4);
@@ -502,6 +502,7 @@ intel_emit_linear_blit(struct intel_context *intel,
    dst_offset += pitch * height;
    size -= pitch * height;
    assert (size < (1 << 15));
+   assert ((size & 3) == 0); /* Pitch must be DWORD aligned */
    if (size != 0) {
       ok = intelEmitCopyBlit(intel, 1,
 			     size, src_bo, src_offset, I915_TILING_NONE,