sdl12-compat: Bind a multisample-resolved READ FBO for OpenGL Scaling

SDL Commits
1 
From 513315b5f21f3421f418e6ea0623636d215adcf5 Mon Sep 17 00:00:00 2001
From: David Gow <[EMAIL REDACTED]>
Date: Thu, 20 May 2021 21:24:23 +0800
Subject: [PATCH] Bind a multisample-resolved READ FBO for OpenGL Scaling

It's possible to glReadPixels() from the default framebuffer if it's
multisampled, but not from an FBO. Work around this by binding last
frame's resolved framebuffer to GL_READ_FRAMEBUFFER if multisampling is
enabled. This could result in bugs from the framebuffer being old, but
nevertheless seems to be an improvement over the existing implementation
(and having to do a resolve on every BindFramebuffer() could potentially
be slow, I guess, though I haven't tested it).
---
 src/SDL12_compat.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/src/SDL12_compat.c b/src/SDL12_compat.c
index 45da6b0..a72bc22 100644
--- a/src/SDL12_compat.c
+++ b/src/SDL12_compat.c
@@ -3316,7 +3316,21 @@ glBindFramebuffer_shim_for_scaling(GLenum target, GLuint name)
 {
     /* OpenGLLogicalScalingFBO will be zero if we aren't scaling, making this use the default. */
     /*if ((name == 0) && (OpenGLLogicalScalingFBO != 0)) { SDL20_Log("OVERRIDE WINDOW FRAMEBUFFER FOR SCALING!"); }*/
-    OpenGLFuncs.glBindFramebuffer(target, (name == 0) ? OpenGLLogicalScalingFBO : name);
+    /* The default framebuffer can be read from in OpenGL, even if it's multisampled. Framebuffers can't.
+       We therefore bind the resolved framebuffer to GL_READ_BUFFER if it's set. This will be one frame
+       old, but this is rarely a problem. If it turns out to cause issues, we could force a resolve at
+       this point. */
+    if (OpenGLLogicalScalingMultisampleFBO && (target == GL_READ_FRAMEBUFFER)) {
+        OpenGLFuncs.glBindFramebuffer(target, (name == 0) ? OpenGLLogicalScalingMultisampleFBO : name);
+    } else if (target == GL_DRAW_FRAMEBUFFER) {
+        OpenGLFuncs.glBindFramebuffer(target, (name == 0) ? OpenGLLogicalScalingFBO : name);
+    } else if (!OpenGLLogicalScalingMultisampleFBO) {
+        OpenGLFuncs.glBindFramebuffer(target, (name == 0) ? OpenGLLogicalScalingFBO : name);
+    } else {
+        /* assume it's GL_FRAMEBUFFER, but we need separate read and draw buffers */
+        OpenGLFuncs.glBindFramebuffer(GL_READ_FRAMEBUFFER, (name == 0) ? OpenGLLogicalScalingMultisampleFBO : name);
+        OpenGLFuncs.glBindFramebuffer(GL_DRAW_FRAMEBUFFER, (name == 0) ? OpenGLLogicalScalingFBO : name);
+    }
 }
 
 
@@ -3371,9 +3385,10 @@ InitializeOpenGLScaling(const int w, const int h)
             OpenGLFuncs.glDeleteFramebuffers(1, &OpenGLLogicalScalingMultisampleFBO);
             OpenGLLogicalScalingMultisampleFBO = OpenGLLogicalScalingMultisampleColor = OpenGLLogicalScalingMultisampleDepth = 0;
         }
+        OpenGLFuncs.glBindFramebuffer(GL_DRAW_FRAMEBUFFER, OpenGLLogicalScalingFBO);
+        OpenGLFuncs.glBindFramebuffer(GL_READ_FRAMEBUFFER, OpenGLLogicalScalingMultisampleFBO);
     }
 
-    OpenGLFuncs.glBindFramebuffer(GL_FRAMEBUFFER, OpenGLLogicalScalingFBO);
 
     OpenGLFuncs.glViewport(0, 0, w, h);
     OpenGLFuncs.glScissor(0, 0, w, h);