gui: improved FPGAViewWidget::paintGL() performance

Profiling revealed that memcpy() in QOpenGLBuffer::allocate() had been taking
the most time during paintGL() calls. I've been able to take the CPU usage
down to about 1/4 of its previous values by caching elements in VBOs and
updating them only after subsequent calls to renderGraphicElement().

Signed-off-by: Mateusz Zalega <mateusz@appliedsourcery.com>

common/nextpnr.h

@@ -157,11 +157,25 @@ struct GraphicElement
 
     enum style_t
     {
+        STYLE_GRID,
         STYLE_FRAME,    // Static "frame". Contrast between STYLE_INACTIVE and STYLE_ACTIVE
         STYLE_HIDDEN,   // Only display when object is selected or highlighted
         STYLE_INACTIVE, // Render using low-contrast color
         STYLE_ACTIVE,   // Render using high-contast color
 
+        // UI highlight groups
+        STYLE_HIGHLIGHTED0,
+        STYLE_HIGHLIGHTED1,
+        STYLE_HIGHLIGHTED2,
+        STYLE_HIGHLIGHTED3,
+        STYLE_HIGHLIGHTED4,
+        STYLE_HIGHLIGHTED5,
+        STYLE_HIGHLIGHTED6,
+        STYLE_HIGHLIGHTED7,
+
+        STYLE_SELECTED,
+        STYLE_HOVER,
+
         STYLE_MAX
     } style = STYLE_FRAME;