Fix frametiming for kmsgrab and x11grab

This commit computes the time of the next screen capture based on the
current frame's theoretical time point instead of the actual capture
time. This should be slightly more precise and lead to better frame
timing.

src/platform/linux/kmsgrab.cpp

@@ -1203,7 +1203,10 @@ namespace platf {
             std::this_thread::sleep_for(1ns);
             now = std::chrono::steady_clock::now();
           }
-          next_frame = now + delay;
+          next_frame += delay;
+          if (next_frame < now) { // some major slowdown happened; we couldn't keep up
+            next_frame = now + delay;
+          }
 
           std::shared_ptr<platf::img_t> img_out;
           auto status = snapshot(pull_free_image_cb, img_out, 1000ms, *cursor);
@@ -1422,7 +1425,10 @@ namespace platf {
             std::this_thread::sleep_for(1ns);
             now = std::chrono::steady_clock::now();
           }
-          next_frame = now + delay;
+          next_frame += delay;
+          if (next_frame < now) { // some major slowdown happened; we couldn't keep up
+            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

@@ -491,7 +491,10 @@ namespace platf {
           std::this_thread::sleep_for(1ns);
           now = std::chrono::steady_clock::now();
         }
-        next_frame = now + delay;
+	next_frame += delay;
+	if (next_frame < now) { // some major slowdown happened; we couldn't keep up
+	  next_frame = now + delay;
+	}
 
         std::shared_ptr<platf::img_t> img_out;
         auto status = snapshot(pull_free_image_cb, img_out, 1000ms, *cursor);