internal/rendertest: create test suit for drawing operations

Uses app/headless to create a set of test cases for drawing operations, including clipping
textures and transforms. This commit tests for approximate pixel matches, if future changes affect
local drawing operations it will be easy to change the reference images, it thus becomes and
should be an intentional operation if changes lead to local changes in drawn results.

Ideally we should be able to make the tests check for exact pixel matches down the line to ensure
consistent results between platforms.

Signed-off-by: Viktor <viktor.ogeman@gmail.com>

internal/rendertest/doc.go

@@ -0,0 +1,2 @@
+// Package rendertest is intended for testing of drawing ops only.
+package rendertest