Added a test that creates two vertex buffers with different strides and pointers and
tests that the renderer can handle the changing strides and pointers.
The resulting image should be a fat diamond with colored vertices as seen
in the included image.
do not fix stride in renderers
refactor gl_renderers.c
improve test setup for test_gl_renderers
fix attribute offsets when they start at >0 and there is an array buffer
fix gl_renderers.png
do not fix offsets in renderers; fixes test_gl_renderers
GL_UNSAFE_OPTS tweaks
invalidate renderers using stride