drm_hwcomposer: name the texture samplers individually and unroll loop

drm_hwcomposer: name the texture samplers individually and unroll loop

Per GLSL spec: when aggregated into arrays within a shader, samplers can
only be indexed with a "dynamically uniform integra expression", other-
wise results are undefined.

The current fragment shader is not spec compliant because it evaluates
the sampler array in a non-uniform flow control, where the loop may exit
early in a "break" statement depending on the per-pixel alpha value.

BUG=24718688
BUG=chrome-os-partner:46348
TEST=follow bug report and verify GLWorker composites 5 layers correctly

Change-Id: I3a4c4b5aa847a1b5ec32f58f0e8c0b349289f5f5