Fix touch events HiDPI-scaling [TODO: rebase on dev]

Touch events were HiDPI-scaled twice:
 - once because the position (provided as floats between 0 and 1) were
   converted in pixels using the drawable size (not the window size)
 - once due to screen_convert_to_frame_coords()

One possible fix could be to compute the position in pixels from the
window size instead, but this would unnecessarily round the event
position to the nearest window coordinates (instead of drawable
coordinates).

Instead, expose two separate functions to convert to frame coordinates
from either window or drawable coordinates.

Fixes #1536 <#1536>
Refs #15 <#15>
Refs e40532a

app/src/input_manager.c

@@ -427,7 +427,7 @@ convert_mouse_motion(const SDL_MouseMotionEvent *from, struct screen *screen,
     to->inject_touch_event.pointer_id = POINTER_ID_MOUSE;
     to->inject_touch_event.position.screen_size = screen->frame_size;
     to->inject_touch_event.position.point =
-        screen_convert_to_frame_coords(screen, from->x, from->y);
+        screen_convert_window_to_frame_coords(screen, from->x, from->y);
     to->inject_touch_event.pressure = 1.f;
     to->inject_touch_event.buttons = convert_mouse_buttons(from->state);
 
@@ -465,15 +465,15 @@ convert_touch(const SDL_TouchFingerEvent *from, struct screen *screen,
     to->inject_touch_event.pointer_id = from->fingerId;
     to->inject_touch_event.position.screen_size = screen->frame_size;
 
-    int ww;
-    int wh;
-    SDL_GL_GetDrawableSize(screen->window, &ww, &wh);
+    int dw;
+    int dh;
+    SDL_GL_GetDrawableSize(screen->window, &dw, &dh);
 
     // SDL touch event coordinates are normalized in the range [0; 1]
-    int32_t x = from->x * ww;
-    int32_t y = from->y * wh;
+    int32_t x = from->x * dw;
+    int32_t y = from->y * dh;
     to->inject_touch_event.position.point =
-        screen_convert_to_frame_coords(screen, x, y);
+        screen_convert_drawable_to_frame_coords(screen, x, y);
 
     to->inject_touch_event.pressure = from->pressure;
     to->inject_touch_event.buttons = 0;
@@ -503,7 +503,7 @@ convert_mouse_button(const SDL_MouseButtonEvent *from, struct screen *screen,
     to->inject_touch_event.pointer_id = POINTER_ID_MOUSE;
     to->inject_touch_event.position.screen_size = screen->frame_size;
     to->inject_touch_event.position.point =
-        screen_convert_to_frame_coords(screen, from->x, from->y);
+        screen_convert_window_to_frame_coords(screen, from->x, from->y);
     to->inject_touch_event.pressure = 1.f;
     to->inject_touch_event.buttons =
         convert_mouse_buttons(SDL_BUTTON(from->button));
@@ -568,7 +568,8 @@ convert_mouse_wheel(const SDL_MouseWheelEvent *from, struct screen *screen,
 
     struct position position = {
         .screen_size = screen->frame_size,
-        .point = screen_convert_to_frame_coords(screen, mouse_x, mouse_y),
+        .point = screen_convert_window_to_frame_coords(screen,
+                                                       mouse_x, mouse_y),
     };
 
     to->type = CONTROL_MSG_TYPE_INJECT_SCROLL_EVENT;

app/src/screen.c

@@ -579,14 +579,14 @@ screen_handle_window_event(struct screen *screen,
 }
 
 struct point
-screen_convert_to_frame_coords(struct screen *screen, int32_t x, int32_t y) {
+screen_convert_drawable_to_frame_coords(struct screen *screen,
+                                        int32_t x, int32_t y) {
     unsigned rotation = screen->rotation;
     assert(rotation < 4);
 
     int32_t w = screen->content_size.width;
     int32_t h = screen->content_size.height;
 
-    screen_hidpi_scale_coords(screen, &x, &y);
 
     x = (int64_t) (x - screen->rect.x) * w / screen->rect.w;
     y = (int64_t) (y - screen->rect.y) * h / screen->rect.h;
@@ -615,6 +615,13 @@ screen_convert_to_frame_coords(struct screen *screen, int32_t x, int32_t y) {
     return result;
 }
 
+struct point
+screen_convert_window_to_frame_coords(struct screen *screen,
+                                      int32_t x, int32_t y) {
+    screen_hidpi_scale_coords(screen, &x, &y);
+    return screen_convert_drawable_to_frame_coords(screen, x, y);
+}
+
 void
 screen_hidpi_scale_coords(struct screen *screen, int32_t *x, int32_t *y) {
     // take the HiDPI scaling (dw/ww and dh/wh) into account

app/src/screen.h

@@ -126,7 +126,14 @@ screen_handle_window_event(struct screen *screen, const SDL_WindowEvent *event);
 // convert point from window coordinates to frame coordinates
 // x and y are expressed in pixels
 struct point
-screen_convert_to_frame_coords(struct screen *screen, int32_t x, int32_t y);
+screen_convert_window_to_frame_coords(struct screen *screen,
+                                      int32_t x, int32_t y);
+
+// convert point from drawable coordinates to frame coordinates
+// x and y are expressed in pixels
+struct point
+screen_convert_drawable_to_frame_coords(struct screen *screen,
+                                        int32_t x, int32_t y);
 
 // Convert coordinates from window to drawable.
 // Events are expressed in window coordinates, but content is expressed in