Include an optional screen size in the WindowsDesktop spec (#38307)

By default this should remain unspecified, which preserves the
behavior we have today (Teleport uses the size of the user's
browser window).

These new fields allow cluster administrators to configure desktops
in such a way that they always use a particular screen size, ignoring
the size passed from the browser client. This is necessary to
accommodate some applications which require a very specific screen resolution in
order to render correctly.

api/proto/teleport/legacy/types/types.proto

@@ -4995,6 +4995,15 @@ message WindowsDesktopSpecV3 {
   // NonAD marks this desktop as a standalone host that is
   // not joined to an Active Directory domain.
   bool NonAD = 4 [(gogoproto.jsontag) = "non_ad"];
+  // ScreenSize specifies the size of the screen to use for sessions
+  // on this host. In most cases this should be unspecified, in which
+  // case Teleport will fill the browser window.
+  Resolution ScreenSize = 5 [(gogoproto.jsontag) = "screen_size,omitempty"];
+}
+
+message Resolution {
+  uint32 Width = 1 [(gogoproto.jsontag) = "width,omitempty"];
+  uint32 Height = 2 [(gogoproto.jsontag) = "height,omitempty"];
 }
 
 // RegisterUsingTokenRequest is a request to register with the auth server using

api/types/desktop.go

@@ -25,6 +25,11 @@ import (
 	"github.com/gravitational/teleport/api/utils"
 )
 
+const (
+	MaxRDPScreenWidth  = 8192
+	MaxRDPScreenHeight = 8192
+)
+
 // WindowsDesktopService represents a Windows desktop service instance.
 type WindowsDesktopService interface {
 	// ResourceWithLabels provides common resource methods.
@@ -132,6 +137,10 @@ type WindowsDesktop interface {
 	// NonAD checks whether this is a standalone host that
 	// is not joined to an Active Directory domain.
 	NonAD() bool
+	// GetScreenSize returns the desired size of the screen to use for sessions
+	// to this host. Returns (0, 0) if no screen size is set, which means to
+	// use the size passed by the client over TDP.
+	GetScreenSize() (width, height uint32)
 	// Copy returns a copy of this windows desktop
 	Copy() *WindowsDesktopV3
 	// CloneResource returns a copy of the WindowDesktop as a ResourceWithLabels
@@ -179,9 +188,24 @@ func (d *WindowsDesktopV3) CheckAndSetDefaults() error {
 	if err := d.ResourceHeader.CheckAndSetDefaults(); err != nil {
 		return trace.Wrap(err)
 	}
+
+	if d.Spec.ScreenSize != nil {
+		if d.Spec.ScreenSize.Width > MaxRDPScreenWidth || d.Spec.ScreenSize.Height > MaxRDPScreenHeight {
+			return trace.BadParameter("invalid screen size %dx%d (maximum %dx%d)",
+				d.Spec.ScreenSize.Width, d.Spec.ScreenSize.Height, MaxRDPScreenWidth, MaxRDPScreenHeight)
+		}
+	}
+
 	return nil
 }
 
+func (d *WindowsDesktopV3) GetScreenSize() (width, height uint32) {
+	if d.Spec.ScreenSize == nil {
+		return 0, 0
+	}
+	return d.Spec.ScreenSize.Width, d.Spec.ScreenSize.Height
+}
+
 // NonAD checks whether host is part of Active Directory
 func (d *WindowsDesktopV3) NonAD() bool {
 	return d.Spec.NonAD