[client/host] added new asyncronous memory copy

This changes the method of the memory copy from the host application to
the guest. Instead of performing a full copy from the capture device
into shared memory, and then flagging the new frame, we instead set a
write pointer, flag the client that there is a new frame and then copy
in chunks of 1024 bytes until the entire frame is copied. The client
upon seeing the new frame flag begins to poll at high frequency the
write pointer and upon each update copies as much as it can into the
texture.

This should improve latency but also slightly increase CPU usage on the
client due to the high frequency polling.

common/include/common/KVMFR.h

@@ -49,7 +49,7 @@ CursorType;
 
 typedef struct KVMFRCursor
 {
-  uint8_t    flags;       // KVMFR_CURSOR_FLAGS
+  volatile uint8_t flags; // KVMFR_CURSOR_FLAGS
   int16_t    x, y;        // cursor x & y position
 
   uint32_t   version;     // shape version
@@ -65,7 +65,7 @@ KVMFRCursor;
 
 typedef struct KVMFRFrame
 {
-  uint8_t     flags;       // KVMFR_FRAME_FLAGS
+  volatile uint8_t flags;  // KVMFR_FRAME_FLAGS
   FrameType   type;        // the frame data type
   uint32_t    width;       // the width
   uint32_t    height;      // the height
@@ -83,7 +83,7 @@ typedef struct KVMFRHeader
 {
   char        magic[sizeof(KVMFR_HEADER_MAGIC)];
   uint32_t    version;     // version of this structure
-  uint8_t     flags;       // KVMFR_HEADER_FLAGS
+  volatile uint8_t flags;  // KVMFR_HEADER_FLAGS
   KVMFRFrame  frame;       // the frame information
   KVMFRCursor cursor;      // the cursor information
 }