4377:  drawVertices crashes when shader+colorfilter in the paint

Fix memory corruption that is result of static_casting SkFilterShaderContext*
 to SkTriColorShaderContext* and then changing it's state using setup()

SkTriColorShader and it's Context could be wrapped by Compose, 3DShader and
 SkFilterShader, but the link SkTriColorShaderContext -> SkTriColorShader
 (fShader) always remain - so this way, data about triangle is propagated.

BUG=4377

Didn't add new unit test - it would be redundant since the GM vertices update here: https://codereview.chromium.org/1361173003/ is covering the bug. With this patch it passes.

Review URL: https://codereview.chromium.org/1431713003
1 file changed