egl: Fix debug flag for SW composition in Surfaceflinger

The debug.sf.hw property makes it possible to force Surfaceflinger
composition in SW by making it choose a slow EGL config. This feature
was broken in JB, which always loads only the HW renderer.
This change fixes it by loading the SW renderer when Surfaceflinger is
started while the debug property is set to 0.

Change-Id: Ibe44c1e6e7c9059d7be024c36121a42a25450aef
egl: Switch to NOP dispatch table on ReleaseThread

ReleaseThread releases currently bounded context but fails
to set NOP dispatch table.

As per spec its action should be same as calling eglMakeCurrent
with ctx set to EGL_NO_CONTEXT and both draw and read set to
EGL_NO_SURFACE which involves setting NOP dispatch Table as well.

Change-Id: I9494cd7020260a2477db2c4e086a4586cfec0316
egl: Fix memory leak on gl_extensions

Reset gl_extension when loseCurrent is called.

Change-Id: Icdb84fad12db297f3953fdc545b76106cdc4d99c