GRIM/EMI: Call glViewport for apitrace compatibility

glViewport needs to be called once to avoid the apitrace
error: "could not infer drawable size (glViewport never called)".

This fixes the problem that all apitrace framebuffer pictures
had a default size of 32x32 instead of the correct resolution used
in residualvm.

engines/grim/gfx_opengl_shaders.cpp

@@ -405,6 +405,8 @@ byte *GfxOpenGLS::setupScreen(int screenW, int screenH, bool fullscreen) {
 	// Load emergency built-in font
 	loadEmergFont();
 
+	glViewport(0, 0, _screenWidth, _screenHeight);
+
 	glGenTextures(1, &_storedDisplay);
 
 	glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);