Service::Y2R: Support for grayscale decoding of specific formats

Implements unrotated planar YUV 4:2:0 -> RGB24 conversions in Y2R.
Currently only the Y (luma) channel is used, so the results don't
contain color. This will be added in a later PR at some point.

This is enough to get all currently know Moflex videos to decode. (Some
don't display on-screen due to seemingly unrelated reasons.)

Thanks to @archshift for doing the initial implementation which I
cleaned up and then fixed the 8x8 block mode.

src/common/logging/log.h

@@ -65,6 +65,7 @@ enum class Class : ClassType {
     Service_DSP,                ///< The DSP (DSP control) service
     Service_HID,                ///< The HID (User input) service
     Service_SOC,                ///< The SOC (Socket) service
+    Service_Y2R,                ///< The Y2R (YUV to RGB conversion) service
     HW,                         ///< Low-level hardware emulation
     HW_Memory,                  ///< Memory-map and address translation
     HW_LCD,                     ///< LCD register emulation