Add VPX improvements from pion-mediadevices

- Add vpx_image.go: VpxImage wrapper for vpx_image_t with convenient methods
- Move BitrateTracker to vpx package: More specific to VPX codec usage
- Add bitrate_tracker_test.go: Test coverage for VPX-specific bitrate tracking
- Remove generic codec-level BitrateTracker: Replaced by VPX-specific version

These changes improve VPX codec functionality and organization by:
1. Adding image handling utilities specific to VPX
2. Providing better bitrate tracking for VPX codecs
3. Improving code organization by moving VPX-specific code to VPX package

pkg/codec/vpx/bitrate_tracker.go

@@ -1,4 +1,4 @@
-package codec
+package vpx
 
 import (
 	"time"

pkg/codec/vpx/bitrate_tracker_test.go

@@ -1,4 +1,4 @@
-package codec
+package vpx
 
 import (
 	"math"

pkg/codec/vpx/vpx_image.go

@@ -0,0 +1,40 @@
+package vpx
+
+/*
+#cgo pkg-config: vpx
+#include <vpx/vpx_image.h>
+*/
+import "C"
+import "unsafe"
+
+type VpxImage struct {
+	img *C.vpx_image_t
+}
+
+func NewImageFromPtr(ptr *C.vpx_image_t) *VpxImage {
+	return &VpxImage{img: ptr}
+}
+
+func (i *VpxImage) Width() int {
+	return int(i.img.d_w)
+}
+
+func (i *VpxImage) Height() int {
+	return int(i.img.d_h)
+}
+
+func (i *VpxImage) YStride() int {
+	return int(i.img.stride[0])
+}
+
+func (i *VpxImage) UStride() int {
+	return int(i.img.stride[1])
+}
+
+func (i *VpxImage) VStride() int {
+	return int(i.img.stride[2])
+}
+
+func (i *VpxImage) Plane(n int) unsafe.Pointer {
+	return unsafe.Pointer(i.img.planes[n])
+}