- do not use OpenGL function to clear screen in Cocoa backend

In fullscreen mode there is a white flash (of the default background color) on startup
To overcome this an explicit glClear() with flush buffers was used
In order to make video mode switch more generic, any backend specific functionality should be avoided here

src/posix/cocoa/i_video.mm

@@ -52,10 +52,7 @@
 #include "version.h"
 #include "doomerrors.h"
 
-#include "gl/renderer/gl_renderer.h"
 #include "gl/system/gl_framebuffer.h"
-#include "gl/textures/gl_samplers.h"
-
 #include "vulkan/system/vk_framebuffer.h"
 
 
@@ -182,6 +179,12 @@ namespace
 
 @implementation OpenGLCocoaView
 
+- (void)drawRect:(NSRect)dirtyRect
+{
+	[NSColor.blackColor setFill];
+	NSRectFill(dirtyRect);
+}
+
 - (void)resetCursorRects
 {
 	[super resetCursorRects];
@@ -327,9 +330,6 @@ void SetupOpenGLView(CocoaWindow* window)
 	[[glView openGLContext] makeCurrentContext];
 
 	[window setContentView:glView];
-
-	// To be able to use OpenGL functions in SetMode()
-	ogl_LoadFunctions();
 }
 
 } // unnamed namespace
@@ -658,14 +658,6 @@ void SystemGLFrameBuffer::SetMode(const bool fullscreen, const bool hiDPI)
 		SetWindowedMode();
 	}
 
-	const NSSize viewSize = I_GetContentViewSize(m_window);
-
-	glViewport(0, 0, static_cast<GLsizei>(viewSize.width), static_cast<GLsizei>(viewSize.height));
-	glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
-	glClear(GL_COLOR_BUFFER_BIT);
-
-	[[NSOpenGLContext currentContext] flushBuffer];
-
 	[m_window updateTitle];
 
 	if (![m_window isKeyWindow])