OpenGL: draw area resize handle with new immediate mode

The little grabby handle in the corner of an area. Now uses 1 draw call
instead of 6.

Also one version of the (+) icon to show a hidden region. Why do we
have multiple versions of this?

Fixed a harmless signed/unsigned error.

Fixed a GL state error that prematurely disabled blending.

Added imm_draw_filled_circle function, which can be used for drawing
other widgets.

Work toward T49042 and T49043

source/blender/editors/include/BIF_glutil.h

@@ -81,10 +81,8 @@ void glutil_draw_lined_arc(float start, float angle, float radius, int nsegments
 void glutil_draw_filled_arc(float start, float angle, float radius, int nsegments);
 
 /**
- * Draw a circle outline with the given \a radius,
- * starting at angle \a start and arcing through
- * \a angle. The circle is centered at \a x, \a y
- * and drawn in the XY plane.
+ * Draw a circle outline with the given \a radius.
+ * The circle is centered at \a x, \a y and drawn in the XY plane.
  *
  * \param pos The vertex attribute number for position.
  * \param x Horizontal center.
@@ -94,6 +92,18 @@ void glutil_draw_filled_arc(float start, float angle, float radius, int nsegment
  */
 void imm_draw_lined_circle(unsigned pos, float x, float y, float radius, int nsegments);
 
+/**
+ * Draw a filled circle with the given \a radius.
+ * The circle is centered at \a x, \a y and drawn in the XY plane.
+ *
+ * \param pos The vertex attribute number for position.
+ * \param x Horizontal center.
+ * \param y Vertical center.
+ * \param radius The circle's radius.
+ * \param nsegments The number of segments to use in drawing (more = smoother).
+ */
+void imm_draw_filled_circle(unsigned pos, float x, float y, float radius, int nsegments);
+
 /**
  * Returns a float value as obtained by glGetFloatv.
  * The param must cause only one value to be gotten from GL.