Fix unsafety in SkiaShader storage, and texture unit accounting

bug:22390304

Fixes two issues:

1) The max texture unit wasn't large enough to handle the most complex
ComposeShader case (1 for draw primitive, 2 for gradient shader, 1 for
bitmap shader).

2) If a shader isn't supported by SkiaShader::store, the shader data
needs to be explicitly disabled, so we won't read uninitilized data
from it when trying to read shader information out.

Change-Id: I29ee7b7c1e07f67db88c1707bdc857daa305e713

libs/hwui/SkiaShader.cpp

@@ -370,7 +370,11 @@ void SkiaShader::store(Caches& caches, const SkShader& shader, const Matrix4& mo
     if (tryStoreLayer(caches, shader, modelViewMatrix,
             textureUnit, description, &outData->layerData)) {
         outData->skiaShaderType = kLayer_SkiaShaderType;
+        return;
     }
+
+    // Unknown/unsupported type, so explicitly ignore shader
+    outData->skiaShaderType = kNone_SkiaShaderType;
 }
 
 void SkiaShader::apply(Caches& caches, const SkiaShaderData& data) {

libs/hwui/renderstate/TextureState.cpp

@@ -22,7 +22,8 @@ namespace uirenderer {
 const GLenum kTextureUnits[] = {
     GL_TEXTURE0,
     GL_TEXTURE1,
-    GL_TEXTURE2
+    GL_TEXTURE2,
+    GL_TEXTURE3
 };
 
 TextureState::TextureState()
@@ -33,10 +34,13 @@ TextureState::TextureState()
     GLint maxTextureUnits;
     glGetIntegerv(GL_MAX_COMBINED_TEXTURE_IMAGE_UNITS, &maxTextureUnits);
     LOG_ALWAYS_FATAL_IF(maxTextureUnits < kTextureUnitsCount,
-        "At least %d texture units are required!", kTextureUnitsCount);
+            "At least %d texture units are required!", kTextureUnitsCount);
 }
 
 void TextureState::activateTexture(GLuint textureUnit) {
+    LOG_ALWAYS_FATAL_IF(textureUnit >= kTextureUnitsCount,
+            "Tried to use texture unit index %d, only %d exist",
+            textureUnit, kTextureUnitsCount);
     if (mTextureUnit != textureUnit) {
         glActiveTexture(kTextureUnits[textureUnit]);
         mTextureUnit = textureUnit;

libs/hwui/renderstate/TextureState.h

@@ -73,7 +73,7 @@ public:
     void unbindTexture(GLuint texture);
 private:
     // total number of texture units available for use
-    static const int kTextureUnitsCount = 3;
+    static const int kTextureUnitsCount = 4;
 
     TextureState();
     GLuint mTextureUnit;