Fix broken layerCount for some types of textures (#8151)

Currently mLayerCount for RenderTarget is always updated to the number
of depth for attachments, which incurs unintended behaviors for some
types of textures. i.e., 2d array, cubemap array, and 3d textures.

Fix this by updating mLayerCount only for multiview use case.

BUGS=[369165616]

samples/hellostereo.cpp

@@ -224,6 +224,8 @@ int main(int argc, char** argv) {
         app.stereoRenderTarget = RenderTarget::Builder()
                 .texture(RenderTarget::AttachmentPoint::COLOR, app.stereoColorTexture)
                 .texture(RenderTarget::AttachmentPoint::DEPTH, app.stereoDepthTexture)
+                .multiview(RenderTarget::AttachmentPoint::COLOR, eyeCount, 0)
+                .multiview(RenderTarget::AttachmentPoint::DEPTH, eyeCount, 0)
                 .build(*engine);
         app.stereoView->setRenderTarget(app.stereoRenderTarget);
         app.stereoView->setViewport({0, 0, vp.width, vp.height});