Implement conics for NVPR

Implement conics for NVPR. Verified with currently disabled conicpaths
gm, produces mostly same shapes with nvprmsaa4 and msaa4.

BUG=skia:2033

Review URL: https://codereview.chromium.org/694503003
diff --git a/src/gpu/gl/GrGLDefines.h b/src/gpu/gl/GrGLDefines.h
index afcbe81..68d8da8 100644
--- a/src/gpu/gl/GrGLDefines.h
+++ b/src/gpu/gl/GrGLDefines.h
@@ -802,6 +802,7 @@
 #define GR_GL_LINE_TO                                       0x04
 #define GR_GL_QUADRATIC_CURVE_TO                            0x0A
 #define GR_GL_CUBIC_CURVE_TO                                0x0C
+#define GR_GL_CONIC_CURVE_TO                                0x1A
 
 // path parameters
 #define GR_GL_PATH_STROKE_WIDTH                             0x9075