Linux: simplify sleep before next frame snapshot

Before this commit kmsgrab, x11grab, cuda used a "busy" wait that did
not in fact keep the thread busy since it called `sleep_for()`.
The whole waiting period is now slept for at once.

Wlgrab used a true busy wait for a third of the waiting time. This
entailed a rather high CPU usage. The watiting method has been aligned
to the simplified method of the other Linux capture backends.

(Benchmark data can be found in the discussion of PR #2333.)

src/platform/linux/cuda.cpp

@@ -803,11 +803,7 @@ namespace cuda {
         while (true) {
           auto now = std::chrono::steady_clock::now();
           if (next_frame > now) {
-            std::this_thread::sleep_for((next_frame - now) / 3 * 2);
-          }
-          while (next_frame > now) {
-            std::this_thread::sleep_for(1ns);
-            now = std::chrono::steady_clock::now();
+            std::this_thread::sleep_for(next_frame - now);
           }
           next_frame += delay;
           if (next_frame < now) {  // some major slowdown happened; we couldn't keep up

src/platform/linux/kmsgrab.cpp

@@ -1197,11 +1197,7 @@ namespace platf {
           auto now = std::chrono::steady_clock::now();
 
           if (next_frame > now) {
-            std::this_thread::sleep_for((next_frame - now) / 3 * 2);
-          }
-          while (next_frame > now) {
-            std::this_thread::sleep_for(1ns);
-            now = std::chrono::steady_clock::now();
+            std::this_thread::sleep_for(next_frame - now);
           }
           next_frame += delay;
           if (next_frame < now) {  // some major slowdown happened; we couldn't keep up
@@ -1421,11 +1417,7 @@ namespace platf {
           auto now = std::chrono::steady_clock::now();
 
           if (next_frame > now) {
-            std::this_thread::sleep_for((next_frame - now) / 3 * 2);
-          }
-          while (next_frame > now) {
-            std::this_thread::sleep_for(1ns);
-            now = std::chrono::steady_clock::now();
+            std::this_thread::sleep_for(next_frame - now);
           }
 
           if (config::sunshine.min_log_level <= 1) {

src/platform/linux/wlgrab.cpp

@@ -133,12 +133,12 @@ namespace wl {
         auto now = std::chrono::steady_clock::now();
 
         if (next_frame > now) {
-          std::this_thread::sleep_for((next_frame - now) / 3 * 2);
+          std::this_thread::sleep_for(next_frame - now);
         }
-        while (next_frame > now) {
-          now = std::chrono::steady_clock::now();
+        next_frame += delay;
+        if (next_frame < now) {  // some major slowdown happened; we couldn't keep up
+          next_frame = now + delay;
         }
-        next_frame = now + delay;
 
         std::shared_ptr<platf::img_t> img_out;
         auto status = snapshot(pull_free_image_cb, img_out, 1000ms, *cursor);
@@ -263,12 +263,12 @@ namespace wl {
         auto now = std::chrono::steady_clock::now();
 
         if (next_frame > now) {
-          std::this_thread::sleep_for((next_frame - now) / 3 * 2);
+          std::this_thread::sleep_for(next_frame - now);
         }
-        while (next_frame > now) {
-          now = std::chrono::steady_clock::now();
+        next_frame += delay;
+        if (next_frame < now) {  // some major slowdown happened; we couldn't keep up
+          next_frame = now + delay;
         }
-        next_frame = now + delay;
 
         std::shared_ptr<platf::img_t> img_out;
         auto status = snapshot(pull_free_image_cb, img_out, 1000ms, *cursor);

src/platform/linux/x11grab.cpp

@@ -485,11 +485,7 @@ namespace platf {
         auto now = std::chrono::steady_clock::now();
 
         if (next_frame > now) {
-          std::this_thread::sleep_for((next_frame - now) / 3 * 2);
-        }
-        while (next_frame > now) {
-          std::this_thread::sleep_for(1ns);
-          now = std::chrono::steady_clock::now();
+          std::this_thread::sleep_for(next_frame - now);
         }
         next_frame += delay;
         if (next_frame < now) {  // some major slowdown happened; we couldn't keep up
@@ -629,11 +625,7 @@ namespace platf {
         auto now = std::chrono::steady_clock::now();
 
         if (next_frame > now) {
-          std::this_thread::sleep_for((next_frame - now) / 3 * 2);
-        }
-        while (next_frame > now) {
-          std::this_thread::sleep_for(1ns);
-          now = std::chrono::steady_clock::now();
+          std::this_thread::sleep_for(next_frame - now);
         }
         next_frame += delay;
         if (next_frame < now) {  // some major slowdown happened; we couldn't keep up