Express (GLSL expression, possibly known value) pairs as a class

Express (GLSL expression, possibly known value) pairs as a class

Express (GLSL expression, possibly known value) pairs as a class
instead of two variables Introduces GrGLSLExpr<N> to encapsulate
the expression and possibly constant-folded value of the expression.

This simplifies passing of the expressions to functions.

Changes the shaders with following patterns:
        { // Stage 0: Linear Gradient
        vec4 colorTemp = mix(uGradientStartColor_Stage0, uGradientEndColor_Stage0, clamp(vMatrixCoord_Stage0.x, 0.0, 1
        colorTemp.rgb *= colorTemp.a;
-       output_Stage0 = vec4((vColor) * (colorTemp));
+       output_Stage0 = (vColor * colorTemp);
+       }

Previously the vector cast was always added if constant folding was
effective, regardless of the term dimensions. Now the vector upcast is
not inserted in places where it is not needed, ie. when the binary
operator term is of the target dimension.

Also, some parentheses can be omitted. It is assumed that
GrGLSLExpr<N>("string") constructors construct a simple expression or
parenthesized expression.

Otherwise the shader code remains identical.

R=jvanverth@google.com, bsalomon@google.com, robertphillips@google.com

Author: kkinnunen@nvidia.com

Review URL: https://codereview.chromium.org/25048002

git-svn-id: http://skia.googlecode.com/svn/trunk@11690 2bbb7eff-a529-9590-31e7-b0007b416f81