Added a workaround for an AMD driver bug where setCanvas + clear + setCanvas doesn't work.

src/modules/graphics/opengl/Graphics.cpp

@@ -438,6 +438,27 @@ void Graphics::reset()
 	origin();
 }
 
+void Graphics::dummyDraw()
+{
+	gl.prepareDraw();
+
+	// Draw a triangle using a single vertex position indexed 3 times. The
+	// triangle will be degenerate and won't be rasterized (so the pixel shader
+	// won't be run), and an index array with a single vertex is used so the
+	// vertex shader can't prevent the triangle from being degenerate by using
+	// gl_VertexID.
+	// NOTE: This code will have to be changed to use a vertex and index buffer,
+	// for it to work with a Core Profile GL3 backend. But the bug should be
+	// re-tested in that case as well.
+	const GLfloat vertex[] = {0.0f, 0.0f};
+	const GLushort indices[] = {0, 0, 0};
+
+	gl.useVertexAttribArrays(ATTRIBFLAG_POS);
+	glVertexAttribPointer(ATTRIB_POS, 2, GL_FLOAT, GL_FALSE, 0, vertex);
+
+	gl.drawElements(GL_TRIANGLES, 3, GL_UNSIGNED_SHORT, indices);
+}
+
 void Graphics::clear(Colorf c)
 {
 	Colorf nc = Colorf(c.r/255.0f, c.g/255.0f, c.b/255.0f, c.a/255.0f);
@@ -446,6 +467,9 @@ void Graphics::clear(Colorf c)
 
 	glClearColor(nc.r, nc.g, nc.b, nc.a);
 	glClear(GL_COLOR_BUFFER_BIT | GL_STENCIL_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
+
+	if (gl.bugs.clearRequiresDummyDraw)
+		dummyDraw();
 }
 
 void Graphics::clear(const std::vector<OptionalColorf> &colors)
@@ -510,6 +534,9 @@ void Graphics::clear(const std::vector<OptionalColorf> &colors)
 		else
 			glDrawBuffer(GL_COLOR_ATTACHMENT0);
 	}
+
+	if (gl.bugs.clearRequiresDummyDraw)
+		dummyDraw();
 }
 
 void Graphics::discard(const std::vector<bool> &colorbuffers, bool stencil)