graphics: move captures off-thread when streaming (#885)

## Link to GitHub Issue or related Pull Request, if one exists
#0

## Description of change

Moves the API capture readback off the game's Present thread while a
video stream client is connected.

The readback is a `LockRect` plus a memcpy of the whole back buffer,
roughly 635us at 720p and 1270us at 1080p. On the Present thread that
comes out of the game's frame budget: TDJ (at 120Hz) dropped to 117fps
with a 60fps stream running, and reading on a pool thread instead gave
the full 120 back.

Only streaming takes the off-thread path, gated on a new
`capture_pump::screen_claimed()`.

Screenshots, one-off API captures, and `THREAD_BAN` games all keep the
existing inline read for compat reasons. A pool thread in `LockRect`
while the Present thread sat inside `GetRenderTargetData` deadlocks DDR
X2 for example.

`CLAIMED[]` becomes `std::atomic<bool>` so the capture path does not
take a lock on the Present thread. The read pool has a single worker so
frames cannot be enqueued out of order, and both capture pools are never
destroyed so a late read cannot queue onto a torn-down pool.

The capture pipeline itself is unchanged: `GetRenderTargetData` is still
synchronous on the Present thread.

## Testing
DDR X2
World
IIDX TDJ
SDVX VM

src/spice2x/launcher/options.cpp

@@ -1745,7 +1745,7 @@ static const std::vector<OptionDefinition> OPTION_DEFINITIONS = {
     },
     {
         // APIStreamEnable
-        .title = "API Video Stream Server Enable",
+        .title = "API Video Stream Server Enable (EXPERIMENTAL)",
         .name = "apistream",
         .desc = "Serves the mirrored screen as a video stream, on the API port plus two; "
             "alternative to API screen capture. Requires -api.\n\n"
@@ -1754,7 +1754,8 @@ static const std::vector<OptionDefinition> OPTION_DEFINITIONS = {
             "Parameters: screen (0-3), fps (1-60, default 30), q (1-100, default 70).\n\n"
             "Example with -api 1337: http://host:1339/stream.h264?fps=30&q=70\n\n"
             "VIEW ONLY - touch input still requires -api. "
-            "No password protection or encryption of any kind; video sent in the clear!",
+            "No password protection or encryption of any kind; video sent in the clear!\n\n"
+            "Streaming is known to cause older games to hang and crash.",
         .type = OptionType::Bool,
         .category = "Companion & API",
     },