Create a dedicate code path to convert YUV frames to numpy for speed

This avoids the use of hstack, which inevitiably copies the data to a
new memory locaiton.

The speed up is small, but measurable.

I can go from 185 fps decoding of a ~3000 x 2000  video to 200 fps
decoding.

av/video/frame.pyx

@@ -6,7 +6,7 @@ from av.enum cimport define_enum
 from av.error cimport err_check
 from av.utils cimport check_ndarray, check_ndarray_shape
 from av.video.format cimport VideoFormat, get_pix_fmt, get_video_format
-from av.video.plane cimport VideoPlane
+from av.video.plane cimport VideoPlane, YUVPlanes
 
 
 cdef object _cinit_bypass_sentinel
@@ -265,11 +265,21 @@ cdef class VideoFrame(Frame):
         if frame.format.name in ('yuv420p', 'yuvj420p'):
             assert frame.width % 2 == 0
             assert frame.height % 2 == 0
-            return np.hstack((
-                useful_array(frame.planes[0]),
-                useful_array(frame.planes[1]),
-                useful_array(frame.planes[2])
-            )).reshape(-1, frame.width)
+            # Fast path for the case that the entire YUV data is contiguous
+            if (
+                frame.planes[0].line_size == frame.planes[0].width and
+                frame.planes[1].line_size == frame.planes[1].width and
+                frame.planes[2].line_size == frame.planes[2].width
+            ):
+                yuv_planes = YUVPlanes(frame, 0)
+                return useful_array(yuv_planes).reshape(frame.height * 3 // 2, frame.width)
+            else:
+                # Otherwise, we need to copy the data through the use of np.hstack
+                return np.hstack((
+                    useful_array(frame.planes[0]),
+                    useful_array(frame.planes[1]),
+                    useful_array(frame.planes[2])
+                )).reshape(-1, frame.width)
         elif frame.format.name == 'yuyv422':
             assert frame.width % 2 == 0
             assert frame.height % 2 == 0

av/video/plane.pxd

@@ -6,3 +6,7 @@ cdef class VideoPlane(Plane):
 
     cdef readonly size_t buffer_size
     cdef readonly unsigned int width, height
+
+
+cdef class YUVPlanes(VideoPlane):
+    pass

av/video/plane.pyx

@@ -37,3 +37,20 @@ cdef class VideoPlane(Plane):
         """
         def __get__(self):
             return self.frame.ptr.linesize[self.index]
+
+
+cdef class YUVPlanes(VideoPlane):
+    def __cinit__(self, VideoFrame frame, int index):
+        if index != 0:
+            raise RuntimeError("YUVPlanes only supports index 0")
+        if frame.format.name not in ['yuvj420p', 'yuv420p']:
+            raise RuntimeError("YUVPlane only supports yuv420p and yuvj420p")
+        if frame.ptr.linesize[0] < 0:
+            raise RuntimeError("YUVPlane only supports positive linesize")
+        self.width = frame.width
+        self.height = frame.height * 3 // 2
+        self.buffer_size = self.height *  abs(self.frame.ptr.linesize[0])
+        self.frame = frame
+
+    cdef void* _buffer_ptr(self):
+        return self.frame.ptr.extended_data[self.index]