rhi: Add a feature flag for non-fill polygon modes

It's one thing that this is not part of OpenGL ES, but it is optional
even with Vulkan, with some mobile GPUs not offering the feature at all.

Change-Id: I4e2c6642eccb0793e69074b4b6eeb2b7cef3516e
Reviewed-by: Christian Strømme <christian.stromme@qt.io>

src/gui/rhi/qrhi.cpp

@@ -744,6 +744,12 @@ Q_LOGGING_CATEGORY(QRHI_LOG_INFO, "qt.rhi.general")
     it does not map well to all graphics APIs, and it is only meant to provide
     support for special cases anyhow. In practice the feature can be expected to
     be supported with Direct3D 11 and Vulkan.
+
+    \value NonFillPolygonMode Indicates that setting a PolygonMode other than
+    the default Fill is supported for QRhiGraphicsPipeline. A common use case
+    for changing the mode to Line is to get wireframe rendering. This however
+    is not available as a core OpenGL ES feature, and is optional with Vulkan
+    as well as some mobile GPUs may not offer the feature.
  */
 
 /*!
@@ -4324,7 +4330,10 @@ QDebug operator<<(QDebug dbg, const QRhiShaderResourceBindings &srb)
     the fill mode used when rasterizing polygons.  Polygons may be drawn as
     solids (Fill), or as a wire mesh (Line).
 
-    \note OpenGL ES does not support Polygon Mode
+    Support for non-fill polygon modes is optional and is indicated by the
+    QRhi::NonFillPolygonMode feature. With OpenGL ES and some Vulkan
+    implementations the feature will likely be reported as unspported, which
+    then means values other than Fill cannot be used.
 
     \value Fill The interior of the polygon is filled (default)
     \value Line Boundary edges of the polygon are drawn as line segments.

src/gui/rhi/qrhi_p.h

@@ -1697,7 +1697,8 @@ public:
         TextureArrays,
         Tessellation,
         GeometryShader,
-        TextureArrayRange
+        TextureArrayRange,
+        NonFillPolygonMode
     };
 
     enum BeginFrameFlag {

src/gui/rhi/qrhid3d11.cpp

@@ -576,6 +576,8 @@ bool QRhiD3D11::isFeatureSupported(QRhi::Feature feature) const
         return false;
     case QRhi::TextureArrayRange:
         return true;
+    case QRhi::NonFillPolygonMode:
+        return true;
     default:
         Q_UNREACHABLE();
         return false;

src/gui/rhi/qrhigles2.cpp

@@ -1281,6 +1281,8 @@ bool QRhiGles2::isFeatureSupported(QRhi::Feature feature) const
         return caps.geometryShader;
     case QRhi::TextureArrayRange:
         return false;
+    case QRhi::NonFillPolygonMode:
+        return !caps.gles;
     default:
         Q_UNREACHABLE();
         return false;

src/gui/rhi/qrhimetal.mm

@@ -635,6 +635,8 @@ bool QRhiMetal::isFeatureSupported(QRhi::Feature feature) const
         return false;
     case QRhi::TextureArrayRange:
         return false;
+    case QRhi::NonFillPolygonMode:
+        return true;
     default:
         Q_UNREACHABLE();
         return false;

src/gui/rhi/qrhivulkan.cpp

@@ -702,6 +702,8 @@ bool QRhiVulkan::create(QRhi::Flags flags)
     caps.tessellation = physDevFeatures.tessellationShader;
     caps.geometryShader = physDevFeatures.geometryShader;
 
+    caps.nonFillPolygonMode = physDevFeatures.fillModeNonSolid;
+
     if (!importedAllocator) {
         VmaVulkanFunctions afuncs;
         afuncs.vkGetPhysicalDeviceProperties = wrap_vkGetPhysicalDeviceProperties;
@@ -4330,6 +4332,8 @@ bool QRhiVulkan::isFeatureSupported(QRhi::Feature feature) const
         return caps.geometryShader;
     case QRhi::TextureArrayRange:
         return true;
+    case QRhi::NonFillPolygonMode:
+        return caps.nonFillPolygonMode;
     default:
         Q_UNREACHABLE();
         return false;

src/gui/rhi/qrhivulkan_p_p.h

@@ -892,6 +892,7 @@ public:
         bool tessellation = false;
         bool vulkan11OrHigher = false;
         bool geometryShader = false;
+        bool nonFillPolygonMode = false;
     } caps;
 
     VkPipelineCache pipelineCache = VK_NULL_HANDLE;

tests/auto/gui/rhi/qrhi/tst_qrhi.cpp

@@ -407,7 +407,8 @@ void tst_QRhi::create()
             QRhi::TextureArrays,
             QRhi::Tessellation,
             QRhi::GeometryShader,
-            QRhi::TextureArrayRange
+            QRhi::TextureArrayRange,
+            QRhi::NonFillPolygonMode
         };
         for (size_t i = 0; i <sizeof(features) / sizeof(QRhi::Feature); ++i)
             rhi->isFeatureSupported(features[i]);