Convert readSurfacePixels to use a draw context instead of a texture

src/gpu/gl/GrGLGpu.cpp

Index: src/gpu/gl/GrGLGpu.cpp
diff --git a/src/gpu/gl/GrGLGpu.cpp b/src/gpu/gl/GrGLGpu.cpp
index 8b9242cd05f02045fb876584504e683f45aba2dc..ae38ed8779eff4857f5ee65bcca33eccfbe4bc27 100644
--- a/src/gpu/gl/GrGLGpu.cpp
+++ b/src/gpu/gl/GrGLGpu.cpp
@@ -2328,8 +2328,8 @@ bool GrGLGpu::onGetReadPixelsInfo(GrSurface* srcSurface, int width, int height,
     tempDrawInfo->fTempSurfaceDesc.fHeight = height;
     tempDrawInfo->fTempSurfaceDesc.fSampleCnt = 0;
     tempDrawInfo->fTempSurfaceDesc.fOrigin = kTopLeft_GrSurfaceOrigin; // no CPU y-flip for TL.
-    tempDrawInfo->fUseExactScratch = this->glCaps().partialFBOReadIsSlow();
-
+    tempDrawInfo->fTempSurfaceFit = this->glCaps().partialFBOReadIsSlow() ? SkBackingFit::kExact
+                                                                          : SkBackingFit::kApprox;
     // For now assume no swizzling, we may change that below.
     tempDrawInfo->fSwizzle = GrSwizzle::RGBA();