SDL: Update the Renderer dpi_scale on SIZE_CHANGED event (fix #4580)

SDL Commits
1 
From 4077f7a2d9111094129f82f28b7a71afb1878170 Mon Sep 17 00:00:00 2001
From: David Gow <[EMAIL REDACTED]>
Date: Tue, 3 Aug 2021 20:55:45 +0800
Subject: [PATCH] Update the Renderer dpi_scale on SIZE_CHANGED event (fix
 #4580)

The Renderer logical scaling code scales mouse coordinates, and needs to
take the window DPI into account on HIGHDPI windows. However, the
variable which tracks this, renderer->dpi_scale, is set once when the
renderer is created, and then not updated. In the event that the window
is moved to another screen, or the screen DPI otherwise changes, this
will be outdates, and potentially the coordinates will be all wrong.

So let's update the dpi_scale on the SIZE_CHANGED event: it's at least a
possibility that this will be issued on some OSes when DPI changes, and
it's otherwise already handled by SDL_Renderer's event filter.
---
 src/render/SDL_render.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/src/render/SDL_render.c b/src/render/SDL_render.c
index d111e77a1..75adfab5a 100644
--- a/src/render/SDL_render.c
+++ b/src/render/SDL_render.c
@@ -619,6 +619,17 @@ SDL_RendererEventWatch(void *userdata, SDL_Event *event)
                     SDL_SetRenderTarget(renderer, NULL);
                 }
 
+                /* Update the DPI scale if the window has been resized. */
+                if (window && renderer->GetOutputSize) {
+                    int window_w, window_h;
+                    int output_w, output_h;
+                    if (renderer->GetOutputSize(renderer, &output_w, &output_h) == 0) {
+                        SDL_GetWindowSize(renderer->window, &window_w, &window_h);
+                        renderer->dpi_scale.x = (float)window_w / output_w;
+                        renderer->dpi_scale.y = (float)window_h / output_h;
+                    }
+                }
+
                 if (renderer->logical_w) {
                     UpdateLogicalSize(renderer);
                 } else {