Added in check for sized internal format.
Made choice between RED and ALPHA orthogonal to HALF_FLOAT and
HALF_FLOAT_OES.
Enabled rendertarget support on ES 2.0.
Review URL: https://codereview.chromium.org/805033002
It's unclear what params should be used for half float alpha
rendertargets, so they are disabled for the moment.
Review URL: https://codereview.chromium.org/799593002
This allows us to create distance field textures with better precision,
which may help text quality.
BUG=skia:3103
Review URL: https://codereview.chromium.org/762923003
Consolidate read/write funcs in context.
Remove support for reading pixels from a surface that's not a target. It's currently broken and neither used nor tested.
Review URL: https://codereview.chromium.org/648863002
