Support RGB formats with IOSurface client buffer surfaces.

Chrome has started requesting RGB formats for IOSurfaces to support the alpha=false
flag for WebGL context creation.

BUG=chromium:995142

Change-Id: I0c9d994f2d8208e87a0d167ebc0787fc15add08f
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/35389
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Geoff Lang <geofflang@chromium.org>

src/OpenGL/libEGL/Surface.cpp

diff --git a/src/OpenGL/libEGL/Surface.cpp b/src/OpenGL/libEGL/Surface.cpp
index dd912ed..17fc234 100644
--- a/src/OpenGL/libEGL/Surface.cpp
+++ b/src/OpenGL/libEGL/Surface.cpp
@@ -250,6 +250,8 @@
 			return sw::FORMAT_R8;
 		case GL_RG:
 			return sw::FORMAT_G8R8;
+		case GL_RGB:
+			return sw::FORMAT_X8R8G8B8;
 		case GL_BGRA_EXT:
 			return sw::FORMAT_A8R8G8B8;
 		default: