Use GL_RGB as implementation color read format for GL_RGB565

dEQP is very unhappy (and asserts if built in debug) with this since it
has the wrong channel count.

Test: dEQP-GLES3.functional.read_pixels.alignment.*
Change-Id: I35a46ba166d3964091d5e3b3066e3a593952c22a
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/36810
Tested-by: Chris Forbes <chrisforbes@google.com>
Presubmit-Ready: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>

src/OpenGL/libGLESv2/Framebuffer.cpp

diff --git a/src/OpenGL/libGLESv2/Framebuffer.cpp b/src/OpenGL/libGLESv2/Framebuffer.cpp
index 0439bef..88adb8e 100644
--- a/src/OpenGL/libGLESv2/Framebuffer.cpp
+++ b/src/OpenGL/libGLESv2/Framebuffer.cpp
@@ -531,7 +531,7 @@
 		case GL_RGBA4:          return GL_RGBA;
 		case GL_RGB5_A1:        return GL_RGBA;
 		case GL_RGBA8:          return GL_RGBA;
-		case GL_RGB565:         return GL_RGBA;
+		case GL_RGB565:         return GL_RGB;
 		case GL_RGB8:           return GL_RGB;
 		case GL_R8:             return GL_RED;
 		case GL_RG8:            return GL_RG;