Implement DFT partial update bundling

Sending an event every time a DFT packet arrives causes issues
with smoothing in some applications (e.g. Krita) due to the
same data being resent several times as though it is a new event.
This workaround bundles all the updates together and sends them
as one event fixing this problem.

etc/iptsd.conf

@@ -159,3 +159,4 @@
 # PositionExp = -0.7
 # ButtonMinMag = 1000
 # FreqMinMag = 10000
+# AllowSplitEvents = false

src/core/generic/application.hpp

@@ -233,8 +233,15 @@ class Application {
 		if (!m_info.is_touchscreen())
 			return;
 
-		m_dft.input(data);
-		this->process_stylus(m_dft.get_stylus());
+		/* Since all DFT packets update only some of the stylus data, sending an
+		 * event each time a DFT packet arrives causes resending of some stale
+		 * info as though it is current. As a result, the same coordinates,
+		 * pressure, etc. are reported to the system several times. This confuses
+		 * the smoothing algorithms in some software (e.g. Krita), which is why
+		 * we hold off reporting new data to the system until ALL of it is updated.
+		 */
+		if (m_dft.input(data) || m_config.dft_allow_split_events)
+			this->process_stylus(m_dft.get_stylus());
 	}
 
 	/*!

src/core/generic/config.hpp

@@ -65,6 +65,7 @@ class Config {
 	usize dft_mpp2_button_min_mag = 50000;
 	usize dft_mpp2_contact_min_mag = 50000;
 	f64 dft_tilt_distance = 0.6;
+	bool dft_allow_split_events = false;
 
 public:
 	/*!

src/core/generic/dft.hpp

@@ -54,12 +54,20 @@ class DftStylus {
 	 * Loads a DFT window and calculates stylus properties from it.
 	 *
 	 * @param[in] dft The dft window received from the IPTS hardware.
+	 * @return Whether an event should be emitted by this packet.
 	 */
-	void input(const ipts::samples::DftWindow &dft)
+	bool input(const ipts::samples::DftWindow &dft)
 	{
 		switch (dft.type) {
 		case ipts::protocol::dft::Type::Position:
 			this->handle_position(dft);
+			/* This event type is the most important one. It is the first and
+			 * the last one to arrive. It sets both proximity and lift. It is
+			 * also guaranteed to come regularly whenever we have (or just lost)
+			 * proximity. For all these reasons it MUST ALWAYS be let through
+			 * and it is chosen as THE synchronising event.
+			 */
+			return true;
 			break;
 		case ipts::protocol::dft::Type::Button:
 			this->handle_button(dft);
@@ -77,6 +85,8 @@ class DftStylus {
 			// Ignored
 			break;
 		}
+		// Do not emit an event by default.
+		return false;
 	}
 
 	/*!

src/core/linux/config-loader.hpp

@@ -188,6 +188,7 @@ class ConfigLoader {
 		this->get(ini, "DFT", "TiltDistance", m_config.dft_tilt_distance);
 		this->get(ini, "DFT", "Mpp2ContactMinMag", m_config.dft_mpp2_contact_min_mag);
 		this->get(ini, "DFT", "Mpp2ButtonMinMag", m_config.dft_mpp2_button_min_mag);
+		this->get(ini, "DFT", "AllowSplitEvents", m_config.dft_allow_split_events);
 
 		// Legacy options that are kept for compatibility
 		this->get(ini, "DFT", "TipDistance", m_config.stylus_tip_distance);