Fix: Handle SetViewport() and Cls()'s immediate mode by flushing batches before

Problem: SetViewport() and Cls() do their adjustments immediately while all drawing commands (DrawRect, DrawImage...) are batched. This leads to the odd behaviour of viewport adjustments being executed before the batch is rendered. So they happen at another position in the queue then intended. Same for Cls(). So calling Cls() between two "DrawRect()" (which share the same primitive_id) would get executed before the batch renders these two rectangles.

gl2sdlmax2d.mod/gl2sdlmax2d.bmx

@@ -683,6 +683,9 @@ Type TGL2Max2DDriver Extends TMax2DDriver
 	Const PRIMITIVE_TRIANGLE_STRIP:Int = 6
 	Const PRIMITIVE_TEXTURED_TRIANGLE:Int = 7
 
+	Const PRIMITIVE_CLS:Int = 8
+	Const PRIMITIVE_VIEWPORT:Int = 9
+
 	' variables for tracking
 
 	Field vert_index:Int
@@ -895,6 +898,8 @@ Type TGL2Max2DDriver Extends TMax2DDriver
 	End Method
 	
 	Method SetViewport( x, y, w, h )
+		'render what has been batched till now
+		FlushTest( PRIMITIVE_VIEWPORT )
 
 		If x = 0 And y = 0 And w = GraphicsWidth() And h = GraphicsHeight()
 			glDisable( GL_SCISSOR_TEST )
@@ -915,6 +920,9 @@ Type TGL2Max2DDriver Extends TMax2DDriver
 	End Method
 
 	Method Cls()
+		'render what has been batched till now - maybe this happens
+		'with an restricted viewport
+		FlushTest( PRIMITIVE_CLS )
 
 		glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT )