[client] fix some renderer bugs which cause SPICE display corruption

[jacobmcnamee]

See individual commits.

Side note: Much of the EGL texture code seems to uniquely use a convention of "stride in pixels, pitch in bytes" despite a comment in texture.h stating the opposite. The primary source of corruption resolved by this PR (i.e. incorrect pointer math in egl_texBufferStreamUpdate) is only incorrect because the other EGL texture code follows the opposite convention.