GM to stress test texture units/samplers

This test draw the letter "M" with a texture, a rounded rect AA clip and a mask filter. With AA clip turned on it will definitely hit texture units 0, 1 & 3. I'm not sure about #2.

[TomH, 2012-05-01 20:23:54]

Can you throw an GrAssert() or some SkDebugf()s in temporarily to test whether
it hits texture unit #2, so we don't have to guess?

[robertphillips, 2012-05-01 20:44:22]

I was wrong, this hits 0, 2 & 3. We cannot hit all four at once b.c. the glyphs
incorporate the mask into their own mask (so glyph drawing will never hit 1).
The path that does hit 1 (drawWithGPUMaskFilter) is only used for path drawing
(so it will never use 2). As a reminder here is the current allocation:

0: texture
1: mask
2: glyph
3: gpu AA mask
4: spare

[TomH, 2012-05-01 21:18:12]

Ran into appspot server errors when I tried to respond earlier.
LGTM.

[robertphillips, 2012-05-02 13:55:40]

committed as r3814

[robertphillips, 2012-05-02 14:06:47]

Added compiler failure fix in r3815