rhi: gl: Pretend 4x MSAA is always supported when cannot query

With ES 3.0 and up the GL_MAX_SAMPLES query is available and is
guaranteed to be at least 4.

With ES 2.0 there is no such query.

While what we report from supportedSampleCounts() has little
significance with OpenGL (for the window's color buffer it is
anyway the QSurfaceFormat that decides, not QRhi), QRhi users
such as Qt Quick may rely on supportedSampleCounts() returning
real values and may, for example, warn if the value attempted
to use is not in the supported list. (even if MSAA is fully
functional)

Avoid getting such warnings on a pure ES 2.0 implementation
with 4x at least.

Pick-to: 6.7 6.6 6.5
Fixes: QTBUG-120474
Change-Id: I8a34a8bc9ce37ce951fdf7794247e8db44435d41
Reviewed-by: Andy Nichols <andy.nichols@qt.io>

src/gui/rhi/qrhigles2.cpp

@@ -846,8 +846,8 @@ bool QRhiGles2::create(QRhi::Flags flags)
         caps.maxDrawBuffers = 1;
         caps.hasDrawBuffersFunc = false;
         // This does not mean MSAA is not supported, just that we cannot query
-        // the supported sample counts.
-        caps.maxSamples = 1;
+        // the supported sample counts. Assume that 4x is always supported.
+        caps.maxSamples = 4;
     }
 
     caps.msaaRenderBuffer = f->hasOpenGLExtension(QOpenGLExtensions::FramebufferMultisample)