Add TriangleFan pipeline topology

Triangle fan drawing is only supported on some platforms. The feature
flag QRhi::TriangleFanTopology is set accordingly.

In general, TriangleStrip topology will be more efficient on most GPUs.
However, if polygon data is already stored as triangle fans, the CPU
savings may be more significant.

Change-Id: I9704fad751d2119eac8791074f58942dd9315601
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

src/gui/rhi/qrhi.cpp

@@ -563,7 +563,10 @@ Q_LOGGING_CATEGORY(QRHI_LOG_INFO, "qt.rhi.general")
     \value BaseInstance Indicates that instanced draw commands support the \c
     firstInstance argument. When reported as not supported, the firstInstance
     value is ignored and the instance ID starts from 0.
- */
+
+    \value TriangleFanTopology Indicates that QRhiGraphicsPipeline::setTopology()
+    supports QRhiGraphicsPipeline::TriangleFan.
+*/
 
 /*!
     \enum QRhi::BeginFrameFlag
@@ -3123,6 +3126,7 @@ QDebug operator<<(QDebug dbg, const QRhiShaderResourceBindings &srb)
 
     \value Triangles (default)
     \value TriangleStrip
+    \value TriangleFan (only available if QRhi::TriangleFanTopology is supported)
     \value Lines
     \value LineStrip
     \value Points

src/gui/rhi/qrhi_p.h

@@ -1017,6 +1017,7 @@ public:
     enum Topology {
         Triangles,
         TriangleStrip,
+        TriangleFan,
         Lines,
         LineStrip,
         Points
@@ -1426,7 +1427,8 @@ public:
         WideLines,
         VertexShaderPointSize,
         BaseVertex,
-        BaseInstance
+        BaseInstance,
+        TriangleFanTopology
     };
 
     enum BeginFrameFlag {

src/gui/rhi/qrhid3d11.cpp

@@ -469,6 +469,8 @@ bool QRhiD3D11::isFeatureSupported(QRhi::Feature feature) const
         return true;
     case QRhi::BaseInstance:
         return true;
+    case QRhi::TriangleFanTopology:
+        return false;
     default:
         Q_UNREACHABLE();
         return false;

src/gui/rhi/qrhigles2.cpp

@@ -732,6 +732,8 @@ bool QRhiGles2::isFeatureSupported(QRhi::Feature feature) const
         return caps.baseVertex;
     case QRhi::BaseInstance:
         return false; // not in ES 3.2, so won't bother
+    case QRhi::TriangleFanTopology:
+        return true;
     default:
         Q_UNREACHABLE();
         return false;
@@ -1539,6 +1541,8 @@ static inline GLenum toGlTopology(QRhiGraphicsPipeline::Topology t)
         return GL_TRIANGLES;
     case QRhiGraphicsPipeline::TriangleStrip:
         return GL_TRIANGLE_STRIP;
+    case QRhiGraphicsPipeline::TriangleFan:
+        return GL_TRIANGLE_FAN;
     case QRhiGraphicsPipeline::Lines:
         return GL_LINES;
     case QRhiGraphicsPipeline::LineStrip:

src/gui/rhi/qrhimetal.mm

@@ -550,6 +550,8 @@ bool QRhiMetal::isFeatureSupported(QRhi::Feature feature) const
         return true;
     case QRhi::BaseInstance:
         return true;
+    case QRhi::TriangleFanTopology:
+        return false;
     default:
         Q_UNREACHABLE();
         return false;

src/gui/rhi/qrhivulkan.cpp

@@ -3727,6 +3727,8 @@ bool QRhiVulkan::isFeatureSupported(QRhi::Feature feature) const
         return true;
     case QRhi::BaseInstance:
         return true;
+    case QRhi::TriangleFanTopology:
+        return true;
     default:
         Q_UNREACHABLE();
         return false;
@@ -4572,6 +4574,8 @@ static inline VkPrimitiveTopology toVkTopology(QRhiGraphicsPipeline::Topology t)
         return VK_PRIMITIVE_TOPOLOGY_TRIANGLE_LIST;
     case QRhiGraphicsPipeline::TriangleStrip:
         return VK_PRIMITIVE_TOPOLOGY_TRIANGLE_STRIP;
+    case QRhiGraphicsPipeline::TriangleFan:
+        return VK_PRIMITIVE_TOPOLOGY_TRIANGLE_FAN;
     case QRhiGraphicsPipeline::Lines:
         return VK_PRIMITIVE_TOPOLOGY_LINE_LIST;
     case QRhiGraphicsPipeline::LineStrip: