A highly generic Blitter::blitToBuffer() method was used for copying
pixels without requiring any format conversion or scaling.

This change removes Blitter::blitToBuffer() and the unused
blitFromBuffer() methods and implements a copyTo() method which does
straightforward memcpy() of the data.

Also consistently add 'To' and 'From' to the Image class's copy, blit,
and resolve methods to avoid any confusion about which direction the
transfer happens.

Benchmark results:

Run on (48 X 2594 MHz CPU s)
CPU Caches:
  L1 Data 32 KiB (x24)
  L1 Instruction 32 KiB (x24)
  L2 Unified 256 KiB (x24)
  L3 Unified 30720 KiB (x2)
---------------------------------------------------------
Benchmark               Time             CPU   Iterations
---------------------------------------------------------
(LLVM, before)
Triangle/Hello       8.69 ms         7.39 ms          112

(LLVM, after)
Triangle/Hello      0.878 ms        0.454 ms         2133

(Subzero, before)
Triangle/Hello       26.2 ms         24.8 ms           41

(Subzero, after)
Triangle/Hello       1.11 ms        0.432 ms         1339

Bug: b/147967959
Change-Id: I76a103bbb6e582f987ef1c3cc39d17779ae4ac99
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/45650
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>